Remove disabled TabGroup feature
Summary:
This has been commented out since 2014, I doubt it will come back.
This is a big amount of code, maintenance will be easier without it.
Reviewers: KWin, zzag
Reviewed By: KWin, zzag
Subscribers: romangg, graesslin, kwin
Tags: KWin, Documentation
Differential Revision: https://phabricator.kde.org/D23069