In https://github.com/edx/edx-platform/pull/25955 `HiddenDescriptor`
(which was a subclass of `RawDescriptor` with a custom `student_view()`)
was converted to an XBlock. It is used as the `default_class` by the
`CachingDescriptorSystem` classes. However `RawDescriptor` is still
being used by `XMLModuleStore`. This has been replaced by
`HiddenDescriptor` as well.