Impossible to delete domains

[t2d]

There's no reason why admins shouldn't be able to delete a domain when it's not used anymore.

[t2d]

In database currently necessary:

# delete vouchers for each user
MariaDB [mail]> DELETE FROM virtual_vouchers WHERE user_id = 1312;
Query OK, 3 rows affected (0.01 sec)

# delete all users
MariaDB [mail]> DELETE FROM virtual_users WHERE domain_id = 42;
Query OK, 1 row affected (0.00 sec)

# delete domain
MariaDB [mail]> DELETE FROM virtual_domains where name = 'example.de';
Query OK, 1 row affected (0.04 sec)