Excise archive URL as a job concept.

The main reason: jobs can come from either temporary storage (i.e.
Redis) or long-term storage (i.e. CouchDB), but -- for sanity -- not
both.  However, the delay between job completion and archive URL
availability means that keeping archive URLs in jobs requires us to pull
from two different data sources to get a complete job.

I have had to write enough shit like that to know that it never goes
well.