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