We decided to stop allowing new CD Stubs in 2020 (MBS-9848) - while we never actually got to implementing that, it turns out we actually accidentally did it anyway later that year (with https://github.com/mwiencek/musicbrainz-server/commit/e694831613cf65bc16ff8b52a6ddf38a207c50cd) by breaking the system so that it always requires log-in to attach a disc ID (meaning nobody ever could get to the Add CD stub form).

Nobody complained for two years anyway AFAICT, so it seems like a good time to drop the related code.

Users were still allowed to edit CD stubs these two years... but looking at the last modified CD stubs in the DB, nobody did. So no reason to keep that up either.