Fix invalid tabstop

Summary:
Revision 9a59e3fb6cf4c2b272d724520bbfd3ed1a0e5c89 removed the ghns
button, but did not remove the corresponding entry in the tab focus
chain.

Reviewers: KWin, ngraham, zzag

Reviewed By: KWin, zzag

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D23936