Fix invalid tabstop

Summary:
Revision 9a59e3fb 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