mainWindow: Never show fallback app menu

Now that the app menu's action have been moved inside the window and
the menu is only kept around to have GNOME's fallback work correctly,
there is no need for GTK's fallback menu under shells that don't
show the app menu, so filter it out from the decoration layout.

GNOME/polari!53
4 jobs for remove-app-menu in 7 minutes and 7 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Source Check
passed eslint #195836

00:00:47

 
  Build
passed flatpak #195837

00:05:37

 
  Review
passed review #195838

00:00:28

failed stop_review #195839
allowed to fail manual

00:00:15

 
Name Stage Failure
failed
stop_review Review
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