Remove concept of multihead in kcminit

Summary:
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)

Test Plan:
Code grep
Compiles

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D9171