Fixed bookkeeping bug in rmarchive

In master, rmarchive doesn't actually remove archives from the bookkeeping database. The problem is that rmarchive tries to fetch the database entry for the archive using sdb_domain.get_item(archive_id), but get_item actually searches by the item name (which is the same as the filename). I fixed this by using sdb_domain.select to query by archive_id instead.

In my limited testing this seems to work correctly.