messageTray: Remove _lastNotificationRemoved()

No Source subclass used it to do anything special, and with sources
no longer having any UI representation on their own, doing anything
else isn't useful either, so just kill off that hook.

https://bugzilla.gnome.org/show_bug.cgi?id=744850

js/ui/notificationDaemon.js

@@ -551,10 +551,6 @@ const FdoNotificationDaemonSource = new Lang.Class({
         this.destroyNonResidentNotifications();
     },
 
-    _lastNotificationRemoved: function() {
-        this.destroy();
-    },
-
     openApp: function() {
         if (this.app == null)
             return;