CQ: Remove one out of two usages of buildbot_json in try_job_on_rietveld.py

The next step will be to remove the second usage. I'm starting with the one
that seems simpler and easier to remove.

BUG=none

Review URL: https://codereview.chromium.org/103383002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/commit-queue@239056 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed