Remove BasicScreens class

Summary:
After porting all platforms to using AbstractOutput the BasicScreens
class is not needed anymore.

Test Plan: Compiles

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Maniphest Tasks: T11098

Differential Revision: https://phabricator.kde.org/D21800