[win32] Remove MemoryBarrier() fallback implementation

I added these because the older mingw32 toolchain didn't have
MemoryBarrier().  The newer mingw-w64 toolchain however has.
As reported by John Emmas this was causing build failure with
MSVC because of inline issues.  But that reminded me that we
may be taking this path even if the system implements
MemoryBarrier as a function, which is a waste.  So, just remove
it.