mruby-sprintf: remove misleading comment and dead code

The `XXX` comment in `sprintf.c` suggested that not validating
the number of arguments for positional format specifiers was a bug.
However, CRuby's `sprintf` also ignores extra arguments in this
case, making the existing behavior correct.

This commit removes the confusing comment and the disabled code
block that went with it, clarifying the intended behavior and
cleaning up the code.

Co-authored-by: Gemini <gemini@google.com>

mrbgems/mruby-sprintf/src/sprintf.c

@@ -846,15 +846,6 @@ retry:
   }
 
   sprint_exit:
-#if 0
-  /* XXX - We cannot validate the number of arguments if (digit)$ style used.
-   */
-  if (posarg >= 0 && nextarg < argc) {
-    const char *mesg = "too many arguments for format string";
-    if (mrb_test(ruby_debug)) mrb_raise(mrb, E_ARGUMENT_ERROR, mesg);
-    if (mrb_test(ruby_verbose)) mrb_warn(mrb, mesg);
-  }
-#endif
   mrb_str_resize(mrb, result, blen);
 
   return result;