nixos/postfix: undo deprecation of extraConfig, extraMasterConf

I realize that advanced users like to configure services with Nix
attrsets, but I don't think we should remove the option to use the
(configuration) language provided by upstream.

nixos/modules/services/mail/postfix.nix

···
     (mkIf (cfg.dnsBlacklists != []) {
       services.postfix.mapFiles."client_access" = checkClientAccessFile;
     })
-    (mkIf (cfg.extraConfig != "") {
-      warnings = [ "The services.postfix.extraConfig option was deprecated. Please use services.postfix.config instead." ];
-    })
-    (mkIf (cfg.extraMasterConf != "") {
-      warnings = [ "The services.postfix.extraMasterConf option was deprecated. Please use services.postfix.masterConfig instead." ];
-    })
   ]);
 }