We don't remotely support multihead in plasmashell.
Nor do we support two plasmashells running to fake it.
SDDM doesn't either.
Kwin has some code, but we don't check the screen before
choosing the DBus service to use. I doubt it works well.
This kills some code, and a blocking call to klauncher.
(Note, multihead has nothing to do with xrandr spanning screens)