mail: fix rspamd settings

a config keyword has changed which which used to prevent locally sources
mails from being checked again

roles/mail/templates/rspamd/local.d/settings.conf.j2

@@ -9,7 +9,7 @@ localhost_mail {
 			reject = null;
 			greylist = null;
 			"add header" = null;
-			spam = null;
+			"rewrite subject" = null;
 		}
 	}
 }