Since Unity 8 is now being discontinued, how do I uninstall it from my desktop system?
(Please note: This question is not about Unity 7, which is still supported at the time of this writing and should remain installed if present.)
To fully remove Unity 8 from your system running Ubuntu 16.04 LTS or later, follow these instructions:
Open a new Terminal window, and enter the following command (providing your password if requested):
sudo apt purge unity8 ubuntu-system-settings
After the previous command finishes, you may also enter this command to remove all of the unneeded packages that Unity 8 left behind for a cleaner system:
sudo apt autoremove
Reboot your computer to finish the uninstallation process.
(Source: OMG! Ubuntu!)