Summary:
GTK clients check whether _GTK_FRAME_EXTENTS is in _NET_SUPPORTED, so we
don't need this script anymore.

Also, because GTK clients don't set shadow extents, this script
is doing nothing.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: ngraham, jackg, broulik, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D21371