Remove ConfigUpdate handling

Clients no longer apply a rendezvous-server list pushed by the
rendezvous server.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PkpVy2ufxgEjKd3SjXPnJ8

src/common.rs

@@ -737,13 +737,6 @@ async fn test_nat_type_() -> ResultType<bool> {
                 } else {
                     port2 = tnr.port;
                 }
-                if let Some(cu) = tnr.cu.as_ref() {
-                    Config::set_option(
-                        "rendezvous-servers".to_owned(),
-                        cu.rendezvous_servers.join(","),
-                    );
-                    Config::set_serial(cu.serial);
-                }
             }
         } else {
             break;