Remove "Max Threads per Split Point" UCI option

Experimental patch to verify if drop of nps
in endgames at very long TC is due to this.

Suggested by Ronald de Man.

bench: 7451319

src/thread.h

@@ -166,7 +166,6 @@ struct ThreadPool : public std::vector<Thread*> {
 
   bool sleepWhileIdle;
   Depth minimumSplitDepth;
-  size_t maxThreadsPerSplitPoint;
   Mutex mutex;
   ConditionVariable sleepCondition;
   TimerThread* timer;