nixos: recipientDelimiter is no longer a valid configuration option in Postfix 2.11.x or later

Note that this change in Postfix might affect the mlmmj.nix service in
ways I don't fully understand.

nixos/modules/services/mail/mlmmj.nix

@@ -88,7 +88,6 @@ in
 
     services.postfix = {
       enable = true;
-      recipientDelimiter= "+";
       extraMasterConf = ''
         mlmmj unix - n n - - pipe flags=ORhu user=mlmmj argv=${pkgs.mlmmj}/bin/mlmmj-receive -F -L ${spoolDir}/$nextHop
       '';

nixos/modules/services/mail/postfix.nix

@@ -77,8 +77,6 @@ let
       smtpd_tls_key_file = ${cfg.sslKey}
 
       smtpd_use_tls = yes
-
-      recipientDelimiter = ${cfg.recipientDelimiter}
     ''
     + optionalString (cfg.virtual != "") ''
       virtual_alias_maps = hash:/etc/postfix/virtual
@@ -291,14 +289,6 @@ in
         description = "SSL key to use.";
       };
 
-      recipientDelimiter = mkOption {
-        default = "";
-        example = "+";
-        description = "
-          Delimiter for address extension: so mail to user+test can be handled by ~user/.forward+test
-        ";
-      };
-
       virtual = mkOption {
         default = "";
         description = "