Retire OLD_LOCKS option

And make CRITICAL_SECTION locks the only option for Windows.
This guarantees backward compatibility with all the Windows
versions (even XP and older) and an hassle free experience
when compiling for Windows. Tests performed by Ingo and
reported on talkchess confirm there is no speed penalty
against the most modern SRW locks:

http://www.talkchess.com/forum/viewtopic.php?t=41835&start=20

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>