win32: remove xstrdup macro.

Unnecessary (defined in vlc_common.h) and omitting the second operand is a GNU C extension.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

src/win32/thread.c

@@ -866,8 +866,6 @@ static void SelectClockSource (vlc_object_t *obj)
     free (str);
 }
 
-#define xstrdup(str) (strdup(str) ?: (abort(), NULL))
-
 size_t EnumClockSource (vlc_object_t *obj, const char *var,
                         char ***vp, char ***np)
 {