I've installed kubuntu desktop on ubuntu 18.04 with tasksel. How can I uninstall it?
If you installed it via
sudo apt-get install kubuntu-desktop you can uninstall it by
sudo apt-get remove kubuntu-desktop --autoremove
To be safe, run the reconfigure on your login manager.
sudo dpkg-reconfigure gdm3
After that, you might need to reboot your machine.