Stop exposing rb_str_chilled_p

[Feature #20205]

Now that chilled strings no longer appear as frozen, there is no
need to offer an API to check for chilled strings.

We however need to change `rb_check_frozen_internal` to no
longer be a macro, as it needs to check for chilled strings.

variable.c

@@ -1761,7 +1761,7 @@ rb_obj_ivar_set(VALUE obj, ID id, VALUE val)
 VALUE
 rb_vm_set_ivar_id(VALUE obj, ID id, VALUE val)
 {
-    rb_check_frozen_internal(obj);
+    rb_check_frozen(obj);
     rb_obj_ivar_set(obj, id, val);
     return val;
 }