A group with related mailing events cannot be deleted

1. Create a group.
2. Subscribe to it via CiviMail.
3. Try to delete that group.

That group cannot be deleted (and an attempt at deleting throws an SQL error), as there's a foreign key reference from civicrm_mailing_event_subscribe.

We most probably should delete the referencing rows along the way (just as ON CASCADE DELETE would do), but I'm assigning this to Lobo for now for further comments (deleting the referencing rows is a loss of log-kind-of information, after all).