postfix: migrate more options to services.postfix.config

I'm working on deprecating the top-level options, that configure main.cf
upstream in nixpkgs. With this change we stay ahead of the curve.

The `networks_style` option already defaults to `host` since Postfix 3.0,
so I dropped the setting.

```
$ postconf -d | grep networks_style
mynetworks_style = ${{$compatibility_level} <level {2} ? {subnet} : {host}}
````

mail-server/postfix.nix

@@ -248,8 +248,6 @@ in
 
     services.postfix = {
       enable = true;
-      hostname = "${cfg.sendingFqdn}";
-      networksStyle = "host";
       mapFiles."valias" = valiases_file;
       mapFiles."regex_valias" = regex_valiases_file;
       mapFiles."vaccounts" = vaccounts_file;
@@ -271,8 +269,8 @@ in
           "${certificatePath}"
         ];
 
-        # Extra Config
-        mydestination = "";
+        myhostname = cfg.sendingFqdn;
+        mydestination = ""; # disable local mail delivery
         recipient_delimiter = cfg.recipientDelimiter;
         smtpd_banner = "${cfg.fqdn} ESMTP NO UCE";
         disable_vrfy_command = true;