Implements hook_field_storage_purge().
This function deletes data from the database for a single field on an entity.
function field_sql_storage_field_storage_purge($entity_type, $entity, $field, $instance) {
list($id, $vid, $bundle) = entity_extract_ids($entity_type, $entity);
$table_name = _field_sql_storage_tablename($field);
$revision_name = _field_sql_storage_revision_tablename($field);
db_delete($table_name)
->condition('entity_type', $entity_type)
->condition('entity_id', $id)
->execute();
db_delete($revision_name)
->condition('entity_type', $entity_type)
->condition('entity_id', $id)
->execute();
}