fix an unreachable bug fix: remove listeners in _toAddedListeners;

When I read widgets from ui.json, GUIReader builds those ui and setup touch-events and then keeps those touch-events in _toAddedListeners of cc.eventManager until using `addChild(widget);`

In some cases, I need to remove some components in the widget that is binding touch-event before `addChild()`.

But engine forget to remove their associated touch-event from _toAddedListeners.