Remove redundant check in rgengc_check_relation

WB-unprotected objects are always young. This invariant is maintained throughout the code base.
When WB-unprotecting an object, the object gets de-aged.

gc/default/default.c

@@ -6234,7 +6234,7 @@ static void
 rgengc_check_relation(rb_objspace_t *objspace, VALUE obj)
 {
     if (objspace->rgengc.parent_object_old_p) {
-        if (RVALUE_WB_UNPROTECTED(objspace, obj) || !RVALUE_OLD_P(objspace, obj)) {
+        if (!RVALUE_OLD_P(objspace, obj)) {
             rgengc_remember(objspace, objspace->rgengc.parent_object);
             /* It is in the rememberset now, so its remaining children have nothing left
              * to ask for: stop testing them. */