Remove usage of SelfSerializable

`SelfSerializable` has been (almost completely) replaced by PBJ. We can proceed with removing classes that implement it. The one known usage of it that we can't remove at the moment is `CesEvent`, this can be removed once block streams is live.

Once this is all done, `ConstructableRegistry` and other serialization classes can be removed.

Search the repository for SelfSerializable implementations and references, then inspect CesEvent and ConstructableRegistry to separate the deferred block-streams case from removable serialization classes. Confirm that only the known CesEvent usage remains where necessary, remove the eligible classes, and run the project's Java build and tests to verify the cleanup.

Written by the indexing model from the issue text.