Drop MESSAGE_TRAY action mode

The mode is never set after the removal of the bottom tray, so it
no longer makes sense to pass it to allowKeybinding(). We can also
safely remove it from the ActionModes flags altogether without
requiring a synchronized update with gnome-settings-daemon, as
the latter never used any flag value above LOGIN_SCREEN.

js/ui/status/keyboard.js

@@ -242,8 +242,7 @@ const InputSourceManager = new Lang.Class({
         // effect without considerable work to consolidate the usage
         // of pushModal/popModal and grabHelper. See
         // https://bugzilla.gnome.org/show_bug.cgi?id=695143 .
-        if (Main.actionMode == Shell.ActionMode.MESSAGE_TRAY ||
-            Main.actionMode == Shell.ActionMode.POPUP) {
+        if (Main.actionMode == Shell.ActionMode.POPUP) {
             this._modifiersSwitcher();
             return;
         }