/**
 * Implements hook_field_storage_delete_revision().
 */
function {{ machine_name }}_field_storage_delete_revision($entity_type, $entity, $fields) {
  list($id, $vid, $bundle) = entity_extract_ids($entity_type, $entity);

  if (isset($vid)) {
    foreach ($fields as $field_id) {
      $field = field_info_field_by_id($field_id);
      $revision_name = _field_sql_storage_revision_tablename($field);
      db_delete($revision_name)
        ->condition('entity_type', $entity_type)
        ->condition('entity_id', $id)
        ->condition('revision_id', $vid)
        ->execute();
    }
  }
}