Remove main-menubar-ui.xml because it's not used.

Noticed it because it had some really old menu information in it.  Much of which is now programmatically done.

Change-Id: I0a24227df48c00310bc3223f72ba71c00b2c9fc7
Reviewed-on: https://code.wireshark.org/review/5829
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>