fixes #6126 - deleting a subnet that is associated with a host group throws foreign key error

i'm not sure if thats right, as subnets <-> domains have many to many relationships.

what if i deleted a domain by mistake, it would trigger deleting all of its subnets? I think you still want to show an error message acknowledging you want to delete all subnets (and ensure they are not used by other doimains?)