windows support: remove _UWIN define

The _UWIN define causes the mingw headers not to declare deprecated (on
Windows) function names such as open and mkdir. But the code uses these. I
have no idea why this used to work (if it even did), but the original
reason why it was defined seems to have vanished.

mplayer.c

@@ -28,7 +28,6 @@
 #include "talloc.h"
 
 #if defined(__MINGW32__) || defined(__CYGWIN__)
-#define _UWIN 1  /*disable Non-underscored versions of non-ANSI functions as otherwise int eof would conflict with eof()*/
 #include <windows.h>
 #endif
 #include <string.h>