Now that the internal mutter bindings and gnome-shell stopped using
META_KEY_BINDING_REVERSES, and after moving the 'adding shift reverses
the keybinding action' logic to gnome-control-center, we can remove
META_KEY_BINDING_REVERSES from mutter.
Plugin API is broken as this constant is removed from the exported
headers. ABI is broken as using this flag is now a noop.
https://bugzilla.gnome.org/show_bug.cgi?id=732385