src: clean unused code in agent.h

Signed-off-by: gengjiawen <[email protected]>

PR-URL: #25914
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: gengjiawen

src/tracing/agent.h

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