Windows: Kill the dedicated thread for monotonic clock_gettime()

Windows: Kill the dedicated thread for monotonic clock_gettime()

According to Microsoft, anything prior to Vista could provide
inconsistent results for the value of QueryPerformanceCounter() across
different cores. Now that XP is no longer supported, we can drop the
significant overhead of using a dedicated thread pinned to a single core
just to read a timestamp.

The C++11 steady_clock implementation directly wraps
QueryPerformanceCounter(), so if it is good enough for that then it is
good enough for our purposes.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>