postfix: don't cast message_size_limit to string

On unstable this will become a signed integer and there was never a good
reason for this to be a string.

mail-server/postfix.nix

@@ -277,7 +277,7 @@ in
         recipient_delimiter = cfg.recipientDelimiter;
         smtpd_banner = "${cfg.fqdn} ESMTP NO UCE";
         disable_vrfy_command = true;
-        message_size_limit = toString cfg.messageSizeLimit;
+        message_size_limit = cfg.messageSizeLimit;
 
         # virtual mail system
         virtual_uid_maps = "static:5000";