CI: Remove purgeJobs logic, prevent deleting functional jobs

Remove the purgeJobs() logic and call from delete() function.

It turns out that when delete(canary) is called after deploy() in stable, the jobs of the production deployment are deleted, thus losing logs! It is hard to investigate a problem with migrations when the Job and logs are deleted.

With the addition of the appropriate cleanup() logic, purgeJobs() is made superfluous, and in this case, proved to be erroneous.

Relates to #351 (closed)