Fix timeout callback leaks

Make sure that we don't leak oneshot timeout handlers in main.js and
polkitAuthenticationAgent.js by making the callback return false.

https://bugzilla.gnome.org/show_bug.cgi?id=668087

js/ui/main.js

@@ -332,6 +332,7 @@ function _windowRemoved(workspace, window) {
             workspace._lastRemovedWindow = null;
             _queueCheckWorkspaces();
         }
+        return false;
     });
 }