Fixed bookkeeping bug in rmarchive

[alexhuth]

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.