Use CartManager to remove orderItems when they are unavailable

Done, please review!

Here is the removeOrderItem method for reference:

  /**
   * {@inheritdoc}
   */
  public function removeOrderItem(OrderInterface $cart, OrderItemInterface $order_item, $save_cart = TRUE) {
    $order_item->delete();
    $cart->removeItem($order_item);
    $this->eventDispatcher->dispatch(new CartOrderItemRemoveEvent($cart, $order_item), CartEvents::CART_ORDER_ITEM_REMOVE);

    // If this results in an empty cart call the emptyCart method for
    // consistency.
    if ($cart->get('order_items')->isEmpty()) {
      $this->emptyCart($cart, $save_cart);
      return;
    }

    $this->resetCheckoutFlow($cart);
    $this->resetCheckoutStep($cart);
    if ($save_cart) {
      $cart->save();
    }
  }