The method `ZPointer::set_remset_bits`, shown below, is unused now. I think it is unnecessary because it seems that we don't need to set two remembered bits at the same time. It is good to remove this method.

```
inline zpointer ZPointer::set_remset_bits(zpointer ptr) {
  uintptr_t raw_addr = untype(ptr);
  assert(raw_addr != 0, "raw nulls should have been purged in promotion to old gen");
  raw_addr |= ZPointerRemembered0 | ZPointerRemembered1;
  return to_zpointer(raw_addr);
}
```