panel: drop dead code

The _userMenu variable never gets initialized anymore,
so remove some code that only gets run when it's defined.

https://bugzilla.gnome.org/show_bug.cgi?id=651299

js/ui/panel.js

@@ -1054,11 +1054,6 @@ Panel.prototype = {
             let indicator = new constructor();
             this.addToStatusArea(role, indicator, i);
         }
-
-        // PopupMenuManager depends on menus being added in order for
-        // keyboard navigation
-        if (this._userMenu)
-            this._menus.addMenu(this._userMenu.menu);
     },
 
     _insertStatusItem: function(actor, position) {