Remove unnecessary function: `mrb_str_freeze`.

src/string.c

@@ -235,13 +235,6 @@ mrb_str_new_static(mrb_state *mrb, const char *p, size_t len)
   return mrb_obj_value(s);
 }
 
-MRB_API mrb_value
-mrb_str_freeze(mrb_state *mrb, const char *p, size_t len)
-{
-  struct RString *s = str_new_static(mrb, p, len);
-  return mrb_obj_value(s);
-}
-
 static void
 str_decref(mrb_state *mrb, mrb_shared_string *shared)
 {