[ksmserver] Drop multi X screen support

Summary:
We don't support multi X screen anymore in the rest of plasma. Cleans up
a bit of code.

Having multiple monitors via xrandr will still work as before.

Test Plan: Compiles

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21757