refactor(pm/stalwart): remove redundant DNS record

This was added before Stalwart was our live mailserver. At that point,
we could only access it over Tailscale. Nowadays, we can access it
without this record, so it's better to avoid it as it only serves to
make the Tailscale network different to the live internet.

packetmix/systems/teal/stalwart.nix

···
   imports = [ "${project.inputs.nixos-unstable.src}/nixos/modules/services/mail/stalwart-mail.nix" ];
 
   config = {
-    services.headscale.settings.dns.extra_records = [
-      {
-        # mail.freshly.space -> teal
-        name = "mail.freshly.space";
-        type = "A";
-        value = "100.64.0.5";
-      }
-    ];
-
     services.stalwart-mail = {
       enable = true;
       openFirewall = true;