/**
 * Implements hook_field_attach_purge().
 */
function {{ machine_name }}_field_attach_purge($entity_type, $entity, $field, $instance) {
  // find the corresponding data in mymodule and purge it
  if ($entity_type == 'node' && $field->field_name == 'my_field_name') {
    mymodule_remove_mydata($entity->nid);
  }
}