Ubuntu – Remove KDE greeter and install lightdm

You have 2 options: either remove gnome compleately or reinstall it's missing parts.

For removing gnome you will need to know what's left of it on your system (usually apt-get autoremove will take care of that).

Reinstalling gnome only requires that you apt-get install gnome3. This will reinstall everything needed to get gnome working again.

Note: gnome3 is a meta-package, this is so to make installation of gnome easier. This sadly also means that if you remove parts of it it will think that you want to remove all of gnome, so by using gnome you either have a ton of stuff that you'll never use or you don't use gnome at all. Sadly there's no middle path. (Actually there is, but it's major pain.)

The steps in the previous question will remove all of the KDE software, but it won't restore your system back to pre-KDE install state. There are two additional steps that I had to do to restore my system to pre-installation of kubuntu-desktop.

The first was to modify the Plymouth configuration to remove the "Kubuntu" splash screen (this is just cosmetic, but still annoying and unclean, documented in Ubuntu shows 'Kubuntu' on boot

sudo update-alternatives --config default.plymouth
sudo update-initramfs -u

And the second was to restore the lightdm configuration to use the unity-greeter instead of kde-plasma-greeter. First run this command to open the lightdm configuration page:

sudo -H gedit /etc/lightdm/lightdm.conf 

and then edit it to replace these lines:

greeter-session=unity-greeter 
user-session=ubuntu

This last step I cobbled together from these two pages: KDE overrides lightdm logon screen and How to return the login screen of the original unity ubuntu?.