Bug 1354625 - Use mozilla-unified for upstream for all Firefox builds. r=aki, a=NPOTB

Without the use of an upstream the first repo cloned on a machine will
be cached. If a subsequent job references a different repo, it may have
to pull thousands of changesets because those changesets aren't part of
the initially-cloned repo. This is why the --upstream feature to
robustclone exists and is why it uses the mozilla-unified repo.

The mozilla-unified repo is a superset of central, aurora, beta, release,
etc. So by cloning it, you get changesets for all of the repos at the
time of the clone. When a subsequent job comes along and requests
a changeset from a different repo, you likely only need to fetch data
for a handful of changesets, not thousands.

This change adds the upstream url config for all fx_desktop_build jobs,
ensuring it is used. A redundant config entry for the try repo has been
removed as well.

MozReview-Commit-ID: 3EL7aSXS4AG