Remove useless definition of NULL in mbfl_string.h

If NULL is not defined by the platform, mbfl_defs.h already defines it.

Author: alexdowad

ext/mbstring/libmbfl/mbfl/mbfl_string.h

@@ -49,8 +49,4 @@ MBFLAPI extern void mbfl_string_init(mbfl_string *string);
 MBFLAPI extern void mbfl_string_init_set(mbfl_string *string, const mbfl_encoding *encoding);
 MBFLAPI extern void mbfl_string_clear(mbfl_string *string);
 
-#ifndef NULL
-#define NULL 0
-#endif
-
 #endif /* MBFL_STRING_H */