Implement hook_commerce_cart_order_empty to remove discount line items.

Here's my take on how this would look:

/**
 * Implements hook_commerce_cart_order_empty().
 */
function commerce_discount_commerce_cart_order_empty($order) {
  $order_wrapper = entity_metadata_wrapper('commerce_order', $order);
  $line_items_to_delete = array();
  foreach ($order_wrapper->commerce_line_items as $delta => $line_item_wrapper) {
    if ($line_item_wrapper->getBundle() == 'commerce_discount') {
      $line_items_to_delete[] = $line_item_wrapper->getIdentifier();
      $order_wrapper->commerce_line_items->offsetUnset($delta);
    }
  }

  // Delete line items.
  commerce_line_item_delete_multiple($line_items_to_delete);
}

@bojanz Sure thing thanks for the review, seems to be working well so here's a patch.