Removing linux-image kernel causes linux-image-unsigned package to be installed

This is pertaining Ubuntu 18.04 LTS. I tried to save space by removing unneeded Linux kernel, but looks like it will still install other kernel image that is "Unsigned". Why is this?

Here is the log:

root@ubuntu18:~# apt-get remove linux-image-4.18.0-15-generic
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
  linux-image-unsigned-4.18.0-15-generic
Suggested packages:
  fdutils linux-hwe-doc-4.18.0 | linux-hwe-source-4.18.0 linux-hwe-tools
The following packages will be REMOVED:
  linux-image-4.18.0-15-generic linux-modules-extra-4.18.0-15-generic
The following NEW packages will be installed:
  linux-image-unsigned-4.18.0-15-generic
0 upgraded, 1 newly installed, 2 to remove and 8 not upgraded.
Need to get 8,219 kB of archives.
After this operation, 168 MB disk space will be freed.
Do you want to continue? [Y/n]

The original Linux kernel version that ships with the distro appears to be version 4.18. The new version that was just installed was 5.0. So I wonder if there is a reason to "preserve" 4.18.

Wirawan

I solved this by just exiting apt and adding the unsigned image to the end of the apt-get remove command.

I solved (worked around?) this with the linux-purge utility, created for this very purpose.

The command line I used was linux-purge -bc -k 1:

-b to check /boot for kernels not in APT
-c to interactively choose which kernels to remove
-k 1 to keep one earlier version

I then selected both kernels in the text UI (which probably overrode the -k option). I was left with the current/running kernel and the others removed, so it worked perfectly.