No webhook notifications for manifest delete events on the new code path
Context
Problem
We have previously identified that the registry is not emitting webhook notifications for events when those happen on the new code path (#691 (closed)).
We have fixed that for tag creation and deletion events in #682 (closed), but we also need to do so for manifest delete events. Deleting a manifest on the new code path is a DB cascading operation that will also delete any tags. However, because the tag deletion happens behind the scenes on the DB, there is no tag delete event emitted. This is why we need to explicitly issue a manifest delete event.