How can I remove fonts that I never use, from libreoffice and linux in general?

You have several methods. I can list 4...

1. Graphical interface ===================

Firstly you can install font-manager with this command:

sudo aptitude install font-manager

You can disable (So not remove! Disabling makes them disappear for you but other users on that system will still see them) fonts from here. This is what it looks like:

2. Manual removal from Ubuntu Software Center ==============

As an alternative have a search in Ubuntu Software Center for fonts and delete them from there.

3. Manual removal as packages through command line ==============

None english fonts can be removed with this command:

sudo apt-get remove ttf-indic-fonts-core ttf-kacst-one ttf-khmeros-core ttf-lao ttf-punjabi-fonts ttf-takao-pgothic ttf-thai-tlwg ttf-unfonts-core ttf-wqy-microhei

Here is a list of all the font packages. You can add the font package name to the command above to include them for removal.

If at any time you remove a font that you want back just do a

 sudo apt-get install ttf-indic-fonts-core

And the indic fonts will be re-installed (ofc. change the package name to what you want).

4. Manual command line removal ===========================

Besides this you can look at

/etc/fonts/fonts.conf

to see where fonts are stored. Normally this will be in 1 of these locations:

/usr/share/fonts
/usr/local/share/fonts
/home/$USER/.local/share/fonts
/home/$USER/.fonts

With admin privileges you can just remove them. They should then disappear from any location.

It depends on how you installed the font.

Font installed system-wide

If it was installed system-wide (like most fonts are AFAIK) just pull Nautilus (Ubuntu's file manager) as root (Alt+F2 then enter gksudo nautilus) and navigate to /usr/share/fonts/truetype.

then

then

Delete the file and then run sudo fc-cache -f -v in Terminal.

Log out and back in again and it should be deleted.

Font installed for your user only

If it was installed just for you (manual copy-paste into the user ~/.fonts or ~/.local/share/fonts/ directory) then you can just delete the font from those directories and log out and back in again.

And just to be absolutely sure, do a: locate --existing szNameOfFile to remove all other occurrences of the same file so everything is back where it was!

_{Commands courtesy of @Fabby in chat}

If you want to check what fonts are installed on Debian/Ubuntu systems and derivatives you can run this command:

echo `dpkg -l fonts\*|grep ii|awk '{print $2}'|grep -vE 'fonts-(noto$|liberation|dejavu|freefont|hack)'`

This command EXCLUDES fonts which you want to keep. You can put their names within the parentheses regexp, separated by |. I excluded from this list package names like fonts-noto (explicitly), all fonts-liberation packages, and so on. The list of packages installed can look like the following:

fonts-beng fonts-beng-extra fonts-deva fonts-deva-extra fonts-gargi fonts-gubbi fonts-gujr fonts-gujr-extra fonts-kacst fonts-kacst-one fonts-kalapi fonts-khmeros-core fonts-knda fonts-lohit-beng-assamese fonts-lohit-beng-bengali fonts-lohit-deva fonts-lohit-gujr fonts-lohit-knda fonts-lohit-mlym fonts-lohit-orya fonts-lohit-taml fonts-lohit-taml-classical fonts-lohit-telu fonts-mathjax fonts-mlym fonts-nakula fonts-nanum fonts-navilu fonts-noto-cjk fonts-noto-hinted fonts-noto-mono fonts-noto-unhinted fonts-opensymbol fonts-orya fonts-orya-extra fonts-pagul fonts-sahadeva fonts-samyak-deva fonts-samyak-gujr fonts-samyak-mlym fonts-samyak-taml fonts-sarai fonts-smc fonts-symbola fonts-takao-pgothic fonts-taml fonts-telu fonts-telu-extra fonts-tlwg-garuda fonts-tlwg-garuda-ttf fonts-tlwg-kinnari fonts-tlwg-kinnari-ttf fonts-tlwg-laksaman fonts-tlwg-laksaman-ttf fonts-tlwg-loma fonts-tlwg-loma-ttf fonts-tlwg-mono fonts-tlwg-mono-ttf fonts-tlwg-norasi fonts-tlwg-norasi-ttf fonts-tlwg-purisa fonts-tlwg-purisa-ttf fonts-tlwg-sawasdee fonts-tlwg-sawasdee-ttf fonts-tlwg-typewriter fonts-tlwg-typewriter-ttf fonts-tlwg-typist fonts-tlwg-typist-ttf fonts-tlwg-typo fonts-tlwg-typo-ttf fonts-tlwg-umpush fonts-tlwg-umpush-ttf fonts-tlwg-waree fonts-tlwg-waree-ttf fonts-wqy-microhei

As you can see, this is a rather long list, but it's important to INVESTIGATE it and remove any valuable package which may exists on it. Then, this list should be saved to a file, using file redirection (> file):

echo `dpkg -l fonts\*|grep ii|awk '{print $2}'|grep -vE 'fonts-(noto$|liberation|dejavu|freefont|hack)'` >  not-needed-font-packages.txt

Now, having this file you can always restore (install again) all packages which you will get rid of. Before removing the unnecessary packages whose names are collected in that file, you should run the command below, which will test the system response to this potentially destructive operation:

sudo dpkg --remove --dry-run `cat not-needed-font-packages.txt`

This will check what package WILL be removed. As font packages are often dependencies of other packages, watch the list closely to see if some additional packages would be removed. This command is safe - it will ONLY print what is to be done. If there are any packages, like libreoffice or xreader, you will have to force system to remove ONLY specified packages, which WILL break the consistency of the package system and could be dangerous. However, we are about to remove only some font packages, so no real harm could be done.

To remove ONLY specified packages and IGNORE all broken dependencies and requirements of other installed packages, run the command:

sudo dpkg --remove --force-all --dry-run `cat not-needed-font-packages.txt`

This command WILL ignore all errors, but will print them, to allow you to understand what will be slightly broken. If you upgrade the system and some packages that depended on the fonts will be reinstalled, it is possible that those font packages will be installed again.

To finally remove unnecessary packages, remove the --dry-run option, and run the final command:

sudo dpkg --remove --force-all `cat not-needed-font-packages.txt`

Note, that on your system the situation could be different. On my Polish Linux Mint 18.3 Cinnamon workstation all specified packages were removed and the log looked like this:

(Odczytywanie bazy danych ... 269043 pliki i katalogi obecnie zainstalowane.)
Usuwanie pakietu fonts-beng (2:1.2) ...
Usuwanie pakietu fonts-beng-extra (1.0-5) ...
Usuwanie pakietu fonts-deva (2:1.2) ...
Usuwanie pakietu fonts-deva-extra (3.0-3) ...

... lots of another lines like this...

Usuwanie pakietu fonts-lohit-taml-classical (2.5.3-2) ...
Usuwanie pakietu fonts-lohit-telu (2.5.3-1) ...
Usuwanie pakietu fonts-samyak-taml (1.2.2-4) ...
dpkg: fonts-noto-hinted: problemy z zależnościami, usuwanie według żądania:
 fonts-noto zależy od fonts-noto-hinted.

Usuwanie pakietu fonts-noto-hinted (20160116-1) ...
dpkg: fonts-opensymbol: problemy z zależnościami, usuwanie według żądania:
 libreoffice-core zależy od fonts-opensymbol.
 libreoffice-math zależy od fonts-opensymbol.

Usuwanie pakietu fonts-opensymbol (2:102.7+LibO5.1.4-0ubuntu1) ...
dpkg: fonts-symbola: problemy z zależnościami, usuwanie według żądania:
 ttf-ancient-fonts-symbola zależy od fonts-symbola.

Usuwanie pakietu fonts-symbola (2.59-1) ...
dpkg: fonts-mathjax: problemy z zależnościami, usuwanie według żądania:
 libjs-mathjax zależy od fonts-mathjax (>= 2.6.0~).

Usuwanie pakietu fonts-mathjax (2.6.1-1) ...
Przetwarzanie wyzwalaczy pakietu fontconfig (2.11.94-0ubuntu1.1)...

After that, I have the system with ONLY fonts I like. You can check what font packages remained using this command:

dpkg -l fonts\*|grep ^ii|awk '{print $2}'

The result:

fonts-dejavu
fonts-dejavu-core
fonts-dejavu-extra
fonts-freefont-ttf
fonts-hack-ttf
fonts-liberation
fonts-noto

To refresh the font cache, so that applications stop showing the removed fonts, run

fc-cache -fv