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!50
1 job for remove-app-menu in 2 minutes and 32 seconds (queued for 46 minutes and 2 seconds)
Status Name Job ID Coverage
  Build
passed build-polari #64777

00:02:32