Historically we preserve chunk metadata because the old format of the
Continuous Aggregate has the `chunk_id` column in the materialization
hypertable so in order to don't have chunk ids left over there we just
mark it as dropped when dropping chunks.
In #4269 we introduced a new Continuous Aggregate format that don't
store the `chunk_id` in the materialization hypertable anymore so it's
safe to also remove the metadata when dropping chunk and all associated
Continuous Aggregates are in the new format.
Also added a post-update SQL script to cleanup unnecessary dropped chunk
metadata in our catalog.
Closes#6570
5a359ac660