Remove useless __CYGWIN__ checks

From what I can tell, some really early Cygwin versions from around
1998-2000 used to erroneously define _WIN32. This was eventually fixed,
but these days, the `defined(_WIN32) && !defined(__CYGWIN__)` idiom is
unnecessary.

Now, we only check for __CYGWIN__ in xmlexports.h when deciding whether
to use __declspec.