src: clean unused code in agent.h

Signed-off-by: gengjiawen <technicalcute@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/25914
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/tracing/agent.h

@@ -113,7 +113,6 @@ class Agent {
  private:
   friend class AgentWriterHandle;
 
-  static void ThreadCb(void* arg);
   void InitializeWritersOnThread();
 
   void Start();