postfix: remove option to toggle SMTP smuggling workarounnd

It has been default enabled since Postfix 3.9 and can still be configured
from the NixOS option mentioned in the removal warning.

Removing the option makes our interface leaner.

Information is based on https://www.postfix.org/smtp-smuggling.html#long.

default.nix

@@ -982,6 +982,14 @@ in
     };
 
     redis = {
+      configureLocally = mkOption {
+        type = types.bool;
+        default = true;
+        description = ''
+          Whether to provision a local Redis instance.
+        '';
+      };
+
       address = mkOption {
         type = types.str;
         # read the default from nixos' redis module
@@ -1021,21 +1029,6 @@ in
       '';
     };
 
-    smtpdForbidBareNewline = mkOption {
-      type = types.bool;
-      default = true;
-      description = ''
-        With "smtpd_forbid_bare_newline = yes", the Postfix SMTP server
-        disconnects a remote SMTP client that sends a line ending in a 'bare
-        newline'.
-
-        This feature was added in Postfix 3.8.4 against SMTP Smuggling and will
-        default to "yes" in Postfix 3.9.
-
-        https://www.postfix.org/smtp-smuggling.html
-      '';
-    };
-
     sendingFqdn = mkOption {
       type = types.str;
       default = cfg.fqdn;
@@ -1366,5 +1359,8 @@ in
     (lib.mkRemovedOptionModule [ "mailserver" "dkimBodyCanonicalization" ] ''
       DKIM signing has been migrated to Rspamd, which always uses relaxed canonicalization.
     '')
+    (lib.mkRemovedOptionModule [ "mailserver" "smtpdForbidBareNewline" ] ''
+      The workaround for the SMTP Smuggling attack is default enabled in Postfix >3.9. Use `services.postfix.config.smtpd_forbid_bare_newline` if you need to deviate from its default.
+    '')
   ];
 }