nixos/mailman: don't set Postfix hashes

It's likely that a user might want to set multiple values for
relay_domains, transport_maps, and local_recipient_maps, and the order
is significant.  This means that there's no good way to set these
across multiple NixOS modules, and they should probably all be set
together in the user's Postfix configuration.

So, rather than setting these in the Mailman module, just make the
Mailman module check that the values it needs to occur somewhere, and
advise the user on what to set if not.