Drop overly optimistic index

Drop overly optimistic index

The `channel_updates_id_with_scid_dir_blob` index allows the
intermediate-row-fetching logic to be index-only, but there's very
little reason to do so - we now use subqueries to build the exact
set of rows we want, by id, and then fetch various colums. Having
an index that lets us look up those columns without hitting the
regular table is fine, but there's not a ton of cost to hitting the
table by primary key and maintaining yet another index isn't free.