Remove REF_ enum for java.lang.ref.Reference

After JDK-8283415, `j.l.r.Refernce` can not have more direct subclasses.

```
enum ReferenceType {
  ...
  REF_OTHER, // Subclass of java/lang/ref/Reference, but not subclass of one of the classes below
  ...
}
```

This enum value can be removed.