Implements hook_field_storage_delete_revision().
This function actually deletes the data from the database.
function field_sql_storage_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();
}
}
}