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 Job ID Name Coverage
  Source Check
passed #195836
eslint

00:00:47

 
  Build
passed #195837
flatpak

00:05:37

 
  Review
passed #195838
review

00:00:28

failed #195839
allowed to fail manual
stop_review

00:00:15

 
Name Stage Failure
failed
stop_review Review
Using Docker executor with image fedora:29 ...
Pulling docker image fedora:29 ...
Using docker image sha256:26ffec5b4a8ad65083424903b7aa175953329413fe5cc4c0dac6fedbe81f2fbb for fedora:29 ...
Running on runner-d836cf30-project-911-concurrent-0 via gcc150.bak.milne.osuosl.org...
Cloning repository...
Cloning into '/builds/fmuellner/polari'...
Checking out f8ce0078 as remove-app-menu...
fatal: reference is not a tree: f8ce007810950a0953d3d55bfe8675a74238b4d9
ERROR: Job failed: exit code 1