Remove custom mutex implementation

As part of the investigation of the hang caused by an incorrect implementation
of condition_variable in libwinpthread, it was realized that our custom Mutex
implementation is no longer needed. Prior to lazySMP this custom implementation
resulted in a 30% speedup, but now no speed difference can be measured as no
mutex is used on the hot path in lazySMP.

https://github.com/official-stockfish/Stockfish/issues/2291
https://github.com/official-stockfish/Stockfish/issues/2309#issuecomment-533733393  https://github.com/official-stockfish/Stockfish/issues/2309#issuecomment-533737515

The interest of this patch is that it removes platform-specific code, which is
always less tested.

No functional change.

src/misc.cpp

@@ -168,7 +168,7 @@ void dbg_print() {
 
 std::ostream& operator<<(std::ostream& os, SyncCout sc) {
 
-  static Mutex m;
+  static std::mutex m;
 
   if (sc == IO_LOCK)
       m.lock();