Simplify finished search in ponder/infinite mode.

In this rare case (e.g. go infinite on a stalemate),
just spin till ponderhit/stop comes.

The Thread::wait() is a renmant of the old YBWC
code, today with lazy SMP, threads don't need to
wait when outside of their idle loop.

No functional change.

src/uci.cpp

@@ -176,10 +176,8 @@ void UCI::loop(int argc, char* argv[]) {
       if (    token == "quit"
           ||  token == "stop"
           || (token == "ponderhit" && Threads.stopOnPonderhit))
-      {
           Threads.stop = true;
-          Threads.main()->start_searching(true); // Could be sleeping
-      }
+
       else if (token == "ponderhit")
           Threads.ponder = false; // Switch to normal search