Unity shows wrong distribution name

The PPAs you used to install elementary not only provided elementary specific packages, but also patched many standard Ubuntu packages and with your apt-get dist-upgrade command they all get installed and mixed into your system. Which means some of the core packages are not from Ubuntu repository, but from those elementary PPAs.

For example, This ppa https://launchpad.net/~elementary-os/+archive/ubuntu/os-patches provides many packages that are also available in Ubuntu repository. Since the PPA provides those package with higher version (and patched of course), those are preferred by apt and get installed into your system while you did the dist-upgrade command.

For your specific problem, I identified the package which is responsible for showing the distribution name on top-left corner of Unity Panel. It is called base-files. I tested the elementary PPA by installing it and not surprisingly I too got those elementary marks :D.

But to solve this, you just can't remove it, because this is an essential package and Ubuntu needs it. What you need to do is re-installing from Ubuntu repository. Use this command to do so.

sudo apt-get --reinstall install base-files/xenial-updates

This command will install the package from Ubuntu's xenial-updates archive. After a logout and login, you'll see the changes reverted back.

To completely remove Elementary effect from your system, you should go back to Ubuntu versions for all packages. To do so, use ppa-purge (and not apt-add-repository --remove command, which will just remove the repository entry). For example,

sudo ppa-purge ppa:elementary-os/daily 
sudo ppa-purge ppa:elementary-os/os-patches
sudo ppa-purge ppa:elementary-os/testing 
sudo ppa-purge ppa:mpstark/elementary-tweaks-daily

ppa-purge will remove packages from these PPAs and install the ones from Ubuntu archives.

Note: If you already removed the Elementary PPA entries (by deleting the lines from sources.list files) add them again and then use ppa-purge.

To change the information related to your distribution release and codename, edit the file /etc/lsb-release:

gksu gedit /etc/lsb-release

Mine contains the following information:

DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=16.04
DISTRIB_CODENAME=xenial
DISTRIB_DESCRIPTION="Ubuntu 16.04.1 LTS"