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.