Remove multihead from Plasmashell scripting

Summary:
Plasmashell literally never checks the X screen, but we expose it in
this scripting system.

This patch Hardcodes it to the values you'd have on a normal system.

The existing screenCount is unchanged and will still work.

Test Plan: compiles

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9174