kx: drop the advertisement echo

signed_version settles the server's version inside the key exchange, so
an echo of it on the messages after has nothing left to confirm. With it
go RendezvousMessage.kx_advertised, the check the transport ran on each
decrypted frame, and the tests of both.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019aokqJuhjvB3kijXtAg5Ns

src/websocket.rs

@@ -268,13 +268,6 @@ impl WsFramedStream {
         Ok(())
     }
 
-    #[inline]
-    pub fn check_kx_advertised(&mut self, seen: u32) {
-        if let Some(enc) = self.encrypt.as_mut() {
-            enc.check_kx_advertised(seen);
-        }
-    }
-
     #[inline]
     pub fn is_secured(&self) -> bool {
         self.encrypt.is_some()