Moved item deletion to overridable method.

Closes #3524311

Allows me to override the delete method for modules like queue_stats that require the created property when deleting:

  /**
   * Deletes an item from the queue.
   */
  public function deleteItem(Row $row) {
      $queue = $this->queueFactory->get($row->get('queue_name'));
      $item = (object) [
        'item_id' => $row->get('item_id'),
        'created' => $row->get('created'),
      ];
      $queue->deleteItem($item);
  }