include/ruby/util.h: delete unused codes

- util.h includes defines.h,
- ... which includes ruby/backward/2/stdarg.h,
- ... which always defines _.

This `#ifndef _` must never happen.

include/ruby/util.h

@@ -14,27 +14,6 @@
 #include "ruby/internal/dllexport.h"
 #include "ruby/defines.h"
 
-#ifndef _
-#ifdef __cplusplus
-# ifndef  HAVE_PROTOTYPES
-#  define HAVE_PROTOTYPES 1
-# endif
-# ifndef  HAVE_STDARG_PROTOTYPES
-#  define HAVE_STDARG_PROTOTYPES 1
-# endif
-#endif
-#ifdef HAVE_PROTOTYPES
-# define _(args) args
-#else
-# define _(args) ()
-#endif
-#ifdef HAVE_STDARG_PROTOTYPES
-# define __(args) args
-#else
-# define __(args) ()
-#endif
-#endif
-
 RBIMPL_SYMBOL_EXPORT_BEGIN()
 
 #define DECIMAL_SIZE_OF_BITS(n) (((n) * 3010 + 9998) / 9999)