background: don't call destroy() from the destroy handler

We're calling destroy() in a callback of the destroy handler, which
results in an X server crash at login.

https://bugzilla.gnome.org/show_bug.cgi?id=694240

js/ui/background.js

@@ -301,8 +301,6 @@ const Background = new Lang.Class({
 
         this.actor.disconnect(this._destroySignalId);
         this._destroySignalId = 0;
-
-        this.actor.destroy();
     },
 
     _setLoaded: function() {