Remove field storage information when a field instance is purged.
Called from field_purge_instance() to allow the field storage module to remove field instance information when a field instance is being purged.
$instance: The instance being purged.
function hook_field_storage_purge_field_instance($instance) {
db_delete('my_module_field_instance_info')
->condition('id', $instance['id'])
->execute();
}