Revert "fix warn unused"

- anonymous contributor

This reverts commit 7b7d404269.

src/io_thread.cpp

@@ -91,7 +91,6 @@ void zmq::io_thread_t::out_event ()
 
 void zmq::io_thread_t::timer_event (int id_)
 {
-    ((void)id_);
     //  No timers here. This function is never called.
     zmq_assert (false);
 }