Drop superfluous Cygwin-specific usage of __USE_W32_SOCKETS

Drop check for __USE_W32_SOCKETS in xpce/src/h/unix.h.  This is a
Cygwin-specific thingy, and it's *really* outdated.

Drop redundant check for __USE_W32_SOCKETS in xpce/src/msw/msstream.c.
The file isn't built for Cygwin anyway.

Thanks,
Corinna