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

polyglot.ini

@@ -30,7 +30,6 @@ Space = 100
 Aggressiveness = 100
 Cowardice = 100
 Min Split Depth = 0
-Max Threads per Split Point = 5
 Threads = 1
 Idle Threads Sleep = true
 Hash = 128

src/thread.cpp

@@ -88,7 +88,7 @@ Thread::Thread() /* : splitPoints() */ { // Value-initialization bug in MSVC
   maxPly = splitPointsSize = 0;
   activeSplitPoint = NULL;
   activePosition = NULL;
-  idx = Threads.size();
+  idx = Threads.size(); // Starts from 0
 }
 
 
@@ -213,9 +213,8 @@ void ThreadPool::exit() {
 
 void ThreadPool::read_uci_options() {
 
-  maxThreadsPerSplitPoint = Options["Max Threads per Split Point"];
-  minimumSplitDepth       = Options["Min Split Depth"] * ONE_PLY;
-  size_t requested        = Options["Threads"];
+  minimumSplitDepth = Options["Min Split Depth"] * ONE_PLY;
+  size_t requested  = Options["Threads"];
 
   assert(requested > 0);
 
@@ -297,12 +296,12 @@ void Thread::split(Position& pos, const Stack* ss, Value alpha, Value beta, Valu
   activeSplitPoint = &sp;
   activePosition = NULL;
 
-  size_t slavesCnt = 1; // This thread is always included
+  int slavesCnt = 1; // This thread is always included
   Thread* slave;
 
-  while (    (slave = Threads.available_slave(this)) != NULL
-         && ++slavesCnt <= Threads.maxThreadsPerSplitPoint && !Fake)
+  while (!Fake && (slave = Threads.available_slave(this)) != NULL)
   {
+      ++slavesCnt;
       sp.slavesMask |= 1ULL << slave->idx;
       slave->activeSplitPoint = &sp;
       slave->searching = true; // Slave leaves idle_loop()

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;

src/ucioption.cpp

@@ -70,7 +70,6 @@ void init(OptionsMap& o) {
   o["Aggressiveness"]              = Option(100, 0, 200, on_eval);
   o["Cowardice"]                   = Option(100, 0, 200, on_eval);
   o["Min Split Depth"]             = Option(0, 0, 12, on_threads);
-  o["Max Threads per Split Point"] = Option(5, 4,  8, on_threads);
   o["Threads"]                     = Option(1, 1, MAX_THREADS, on_threads);
   o["Idle Threads Sleep"]          = Option(true);
   o["Hash"]                        = Option(32, 1, 16384, on_hash_size);