Remove use of META_KEY_BINDING_REVERSES

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

src/core/keybindings-private.h

@@ -80,9 +80,6 @@ typedef struct
    */
   GSList *combos;
 
-  /* for keybindings that can have shift or not like Alt+Tab */
-  gboolean      add_shift:1;
-
   /* for keybindings that apply only to a window */
   gboolean      per_window:1;