Remove MetaspaceClosure::FLAG_MASK

https://github.com/openjdk/jdk/blob/75a06421e596b0e0006120a13836ec65a3ab9d46/src/hotspot/share/memory/metaspaceClosure.hpp#L130

    address obj() const {
      // In some rare cases (see CPSlot in constantPool.hpp) we store some flags in the lowest
      // 2 bits of a MetaspaceObj pointer. Unmask these when manipulating the pointer.
      uintx p = (uintx)*mpp();
      return (address)(p & (~FLAG_MASK));
    }

The above was needed only to support CPSlot. Now that CPSlot is removed in JDK-8272107, we can remove MetaspaceClosure::FLAG_MASK and MetaspaceClosure::flag_bits()