`IsDistinctFrom` which was added at #34451 is almost same with
`NotEqual`.

Historically subclasses of `Equality` has a special ability, it has
migrated to `equality?` method, so newer class should implement the
method rather than inheriting the `Equality` if want to have an equality
ability, at least `IsDistinctFrom` should not be an equality node
(actually `IsNotDistinctFrom` is almost same with `Equality`, but I'd
not interested to give a special ability to the node which is rarely
used).