Carousel: simplify carousel items selection

We already know that carousel's parent is the carousel element, so we can use it explicitly

js/src/carousel.js

@@ -267,9 +267,7 @@ class Carousel extends BaseComponent {
   }
 
   _getItemIndex(element) {
-    this._items = element && element.parentNode ?
-      SelectorEngine.find(SELECTOR_ITEM, element.parentNode) :
-      []
+    this._items = SelectorEngine.find(SELECTOR_ITEM, this._element)
 
     return this._items.indexOf(element)
   }