Previously I assumed it was useless, however I was wrong.

The method is called by the reloader to give the illusion that
the GC is precise. Meaning a class that will be unloaded is
immediately made invisible without waiting for it to be garbage collected.

This is easy to do up to Ruby 3.0 because `DescendantTracker` keeps
a map of all tracked classes.

However on 3.1 we need to use the inverse strategy, we keep a WeakMap
of all the classes we cleared, and we filter the return value of `descendants`
and `subclasses`.

Since `clear` is private API and is only used when reloading is enabled,
to reduce the performance impact in production mode, we entirely remove
this behavior when `config.cache_classes` is enabled.