remove unused str_new_frozen

src/sprintf.c

@@ -520,7 +520,6 @@ mrb_str_format(mrb_state *mrb, int argc, const mrb_value *argv, mrb_value fmt)
   ++argc;
   --argv;
   mrb_string_value(mrb, &fmt);
-  fmt = mrb_str_new4(mrb, fmt);
   p = RSTRING_PTR(fmt);
   end = p + RSTRING_LEN(fmt);
   blen = 0;