Eliah Kagan's answer to how to remove KDE here (How to remove all traces of KDE installed) is a great first step in removing KDE from a Ubunutu, but I still have remnants of KDE that were not removed in Plymouth and LightDM. How do you remove these?
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:
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?.