code: function onRemove(el, fn) {
    var observer = new MutationObserver(this.createRemoveNodeHandler(el, fn));
  
    observer.observe(el.parentNode, {childList: true});
  }
docs: Run a function when an element is removed from the DOM.
  Note: Observer is removed after the function is run once.
  @param {Element} el    Element to observe.
  @param {Function} fn   Function to execute when element is removed.