Never use the PGBACKUPS_URL from the environment

It's not clear from the history why this was introduced in the first
place, but the only conceivable use I can think of is testing against
a staging version of PGBackups, but that can easily be done by
actually adding the staging add-on to an app.
Merge pull request #1001 from deafbybeheading/no-pgbackups-url-from-env

Never use the PGBACKUPS_URL from the environment