panel: Delegate container destruction to PanelMenu.ButtonBox

Instead of taking care of the PanelMenu.ButtonBox.container
destruction by itself, delegate that to the very object that
created it in the first place: PanelMenu.ButtonBox itself.

js/ui/panel.js

@@ -1158,7 +1158,6 @@ var Panel = new Lang.Class({
         let destroyId = indicator.connect('destroy', emitter => {
             delete this.statusArea[role];
             emitter.disconnect(destroyId);
-            container.destroy();
         });
         indicator.connect('menu-set', this._onMenuSet.bind(this));
         this._onMenuSet(indicator);