Remove SimpleMutex as it no longer provides a benefit over std::mutex on Windows. It used to be that SimpleMutex was better then boost::mutex. Since boost::mutex is no longer used in the code base, SimpleMutex no longer provides a benefit. On Windows, std::mutex is implemented with the more efficient Slim Reader/Write Lock instead of CriticalSection.

It is located in src/mongo/util/concurrency/mutex.h.