ui: Remove fallback app menu

The app menu is in the process of being retired[0], and the shell
stopped displaying it while applications are in the process of
dropping it. It therefore doesn't make sense to always show a
fallback menu in server-side decorations, applications that still
set the menu can rely on GTK+'s own fallback instead.

[0] https://gitlab.gnome.org/GNOME/Initiatives/wikis/App-Menu-Retirement

gnome-shell#624
2 jobs for wip/fmuellner/remove-app-menu in 4 minutes and 2 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #200745
build-mutter

00:01:13

 
  Test
passed #200746
test-mutter

00:02:49