Revert "Fix a race on Limits::ponder"

This reverts commit 5410424e3d.

After the commit pondering is broken, so revert for now. I will
resubmit with a proper fix.

The issue is mine, Joost original code is correct.

No functional change.

src/thread.cpp

@@ -187,7 +187,7 @@ void ThreadPool::start_thinking(Position& pos, StateListPtr& states,
 
   main()->wait_for_search_finished();
 
-  ponder = stopOnPonderhit = stop = false;
+  stopOnPonderhit = stop = false;
   Search::Limits = limits;
   Search::RootMoves rootMoves;