Simplify rb_obj_freeze

The frozen check in `OBJ_FREEZE` makes the separate check redundant,
and immediate values are always frozen, so the old assertion is
unreachable.

object.c

@@ -1370,12 +1370,7 @@ rb_obj_dummy1(VALUE _x, VALUE _y)
 VALUE
 rb_obj_freeze(VALUE obj)
 {
-    if (!OBJ_FROZEN(obj)) {
-        OBJ_FREEZE_RAW(obj);
-        if (SPECIAL_CONST_P(obj)) {
-            rb_bug("special consts should be frozen.");
-        }
-    }
+    OBJ_FREEZE(obj);
     return obj;
 }