messageTray: Always destroy banners when done displaying

Special-casing banners of resident notifications was really a
thinly veiled special case for chat notifications, as those were
still using the old notification actor which coupled the life-time
of the notification to its actor. This is no longer the case, so
we can do the sane thing and destroy banners once they are no
longer needed.

https://bugzilla.gnome.org/show_bug.cgi?id=746343

js/ui/messageTray.js

@@ -2091,10 +2091,7 @@ const MessageTray = new Lang.Class({
         this._pointerInNotification = false;
         this._notificationRemoved = false;
 
-        if (notification.resident)
-            this._bannerBin.remove_actor(this._banner.actor);
-        else
-            this._banner.actor.destroy();
+        this._banner.actor.destroy();
         this._banner = null;
         this.actor.hide();
     },