Fix compilation error with C23

This fallback already defined at ruby/internal/stdbool.h

Author: hsbt

ext/json/ext/generator/generator.h

@@ -6,14 +6,6 @@
 
 #include "ruby.h"
 
-#ifdef HAVE_STDBOOL_H
-#include <stdbool.h>
-#else
-/* This is the fallback definition from Ruby 3.0.5. */
-typedef unsigned char _Bool
-#define bool _Bool
-#endif
-
 #ifdef HAVE_RUBY_RE_H
 #include "ruby/re.h"
 #else