undefined function metatag_metatag_config_delete()

i'm having this error while trying to delete a vocabulary

Fatal error: Call to undefined function metatag_metatag_config_delete() in /drupal/sites/all/modules/metatags/metatag.module on line 627

/**
 * Implements hook_field_attach_delete_bundle().
 */
function metatag_field_attach_delete_bundle($entity_type, $bundle) {
  $instance = $entity_type . ':' . $bundle;
  metatag_metatag_config_delete($instance);
}

that function seem to be defined nowhere