[Logout Greeter] Remove KDM cruft

KDisplayManager::bootOptions only worked with "NewKDM" but KDM is obsolete in Plasma 5.
Creating an instance of KDisplayManager does DBus stuff to find out the DM being used.
Also, kdmrc is dead and this code always returned None anyway.

Differential Revision: https://phabricator.kde.org/D13998