tstest/integration/testcontrol: remove a vestigial unused parameter

Back in the day this testcontrol package only spoke the
nacl-boxed-based control protocol, which used this.

Then we added ts2021, which didn't, but still sometimes used it.

Then we removed the old mode and didn't remove this parameter
in 2409661a0d.

Updates #11585

Change-Id: Ifd290bd7dbbb52b681b3599786437a15bc98b6a5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/integration/testcontrol/testcontrol.go

@@ -832,7 +832,7 @@ func (s *Server) serveMap(w http.ResponseWriter, r *http.Request, mkey key.Machi
 	w.WriteHeader(200)
 	for {
 		if resBytes, ok := s.takeRawMapMessage(req.NodeKey); ok {
-			if err := s.sendMapMsg(w, mkey, compress, resBytes); err != nil {
+			if err := s.sendMapMsg(w, compress, resBytes); err != nil {
 				s.logf("sendMapMsg of raw message: %v", err)
 				return
 			}
@@ -864,7 +864,7 @@ func (s *Server) serveMap(w http.ResponseWriter, r *http.Request, mkey key.Machi
 				s.logf("json.Marshal: %v", err)
 				return
 			}
-			if err := s.sendMapMsg(w, mkey, compress, resBytes); err != nil {
+			if err := s.sendMapMsg(w, compress, resBytes); err != nil {
 				return
 			}
 		}
@@ -895,7 +895,7 @@ func (s *Server) serveMap(w http.ResponseWriter, r *http.Request, mkey key.Machi
 				}
 				break keepAliveLoop
 			case <-keepAliveTimerCh:
-				if err := s.sendMapMsg(w, mkey, compress, keepAliveMsg); err != nil {
+				if err := s.sendMapMsg(w, compress, keepAliveMsg); err != nil {
 					return
 				}
 			}
@@ -1060,7 +1060,7 @@ func (s *Server) takeRawMapMessage(nk key.NodePublic) (mapResJSON []byte, ok boo
 	return mapResJSON, true
 }
 
-func (s *Server) sendMapMsg(w http.ResponseWriter, mkey key.MachinePublic, compress bool, msg any) error {
+func (s *Server) sendMapMsg(w http.ResponseWriter, compress bool, msg any) error {
 	resBytes, err := s.encode(compress, msg)
 	if err != nil {
 		return err