Using the system ("native") menubar works fine since a long time
surely. No reason to keep a fallback possibility.

Change-Id: I0d9ed86c28b0d832c8123b18980740dbf895ec1c
Reviewed-on: https://gerrit.libreoffice.org/54775Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tor Lillqvist <tml@collabora.com>