Simplify ifdefs in measure_time.h

Instead of using "defined(MSC_VER) || defined(__MINGW32__)" to
indicate the windows platform, just check for the _WIN32 define
instead.

Also remove an unused codepath - the removed codepath would
only be used under the condition
"(defined(MSC_VER) || defined(__MINGW32__)) && !defined(_WIN32)",
and I'm not aware of any environment with MSVC or MinGW that
doesn't define _WIN32, thus this codepath never was used.