Rmilter should not be enabled with Rspamd by default

[pacien]

Issue description

Rmilter is implicitly enabled (here) with Rspamd despite the former being deprecated.

This triggers a deprecation warning since release-18.09 which was hard to trace:

warning: config.services.rmilter' is deprecated, rmilter' deprecated and unsupported by upstream, and will be removed from next releases. Use built-in rspamd milter instead.

Rmilter should instead either be completely disabled by default, or conditionally disabled (with versionAtLeast config.system.stateVersion "18.09").

CC package maintainers: @avnik @fpletz

Steps to reproduce

Set

  services.rspamd.enable = true;

And obtain said warning about Rmilter.

Technical details

Please run nix-shell -p nix-info --run "nix-info -m" and paste the
results.

 - system: `"x86_64-linux"`
 - host os: `Linux 4.14.70, NixOS, 18.03.133280.862fb5215f0 (Impala)`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.0.4`
 - channels(root): `"nixos-18.09"`
 - channels(kea): `""`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs`

[avnik]

At least master haven't any references from rspamd to rmilter.
We definelly had have code, which enable rmilter, when rspamd enabled. But it removed now.
I looking into it (because it a bug, but I also want decomission rmilter as well ;)

[avnik]

I bet, that I found it ;)
https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/mail/rmilter.nix#L55

I feel it can be safely defaulted to false (at least in master)

@fpletz @samueldr folks, what your thoughs about this?