User delete only unsubscribes users from required lists

At present, mailchimp_user() only unsubscribes a user from each list in _mailchimp_get_required_lists() on user delete. This means that if a list is opt-in or opt-out the user is not unsubscribed on account deletion.

I would say that there should at least be a config option to unsubscribe users from all lists, not just required ones, on account delete, and would argue that this should be the default option.