Acts when a field instance is being purged.
In field_purge_instance(), after the field instance has been removed from the database, the field storage module has had a chance to run its hook_field_storage_purge_instance(), and the field info cache has been cleared, this hook is invoked on all modules to allow them to respond to the field instance being purged.
$instance: The instance being purged.
function hook_field_purge_instance($instance) {
db_delete('my_module_field_instance_info')
->condition('id', $instance['id'])
->execute();
}