Remove GTK KCM

Summary:
After D26583 GTK KCM is no longer needed, the functionality
is now in Application Style KCM. Also some outdated files (such as readme and changelog) were removed.

Test Plan:

1. Delete $DEV_DIR/usr/lib/plugins/kcm_kdegtkconfig.so from possible previous builds+installs
2. Build, run system settings with prefix
3. Check, that the only loaded GTK KCM is the old one

Reviewers: mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T10611

Differential Revision: https://phabricator.kde.org/D26593