Remove obsolete cygwin.h hack

The version being checked for is 20 years old.
Reviewed-by: Marco Atzeri <marco.atzeri@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/aa266ede-baaa-f4e6-06cf-5b1737610e9a%402ndquadrant.com

src/include/port/cygwin.h

 /* src/include/port/cygwin.h */
 
-#include <cygwin/version.h>
-
-/*
- * Check for b20.1 and disable AF_UNIX family socket support.
- */
-#if CYGWIN_VERSION_DLL_MAJOR < 1001
-#undef HAVE_UNIX_SOCKETS
-#endif
-
 #ifdef BUILDING_DLL
 #define PGDLLIMPORT __declspec (dllexport)
 #else