Comment 2 for bug 1153827

Originally InstanceMetadata and InstanceSystemMetadata couldn't be deleted because that data was needed to send out notifications for deleted instances (https://code.launchpad.net/bugs/1021830). But since no-db-compute a instance object is passed in pre-deletion instead (https://review.openstack.org/#/c/20354/2/nova/notifications.py). So now we can go back to deleting InstanceSystemMetadata when deleting an instance.