APIv2: Remove unused SOURCE_CONVERSATION_DELETED + ITEM_SEEN #7788
Description
SOURCE_CONVERSATION_DELETED has been superseded by SOURCE_CONVERSATION_TRUNCATED and ITEM_SEEN by SOURCE_CONVERSATION_SEEN.
SOURCE_CONVERSATION_DELETED is inferior because it doesn't have an upper_bound and therefore risks deleting more than the user intended, and ITEM_SEEN is O(N) while it's replacement is O(1).
Once they're unused, we should remove them. There's no urgency to this because APIv2 is not a public stable API, so we can remove things from it as we desire, regardless of whether it was released. Still, might be nice to include in the next release so there's just less code.