repo-updater: remove log, cleanup comments in purge worker

Created by: sashaostrikov

Possible follow-up to https://github.com/sourcegraph/sourcegraph/pull/33276

Success log will spam a lot during initial run against 900k repos, but I'm not 100% sure that we want to miss repo names which were purged.

Another way is to add a label to success monitoring, but it is a less convenient way of looking for repo names which were purged.

Test plan

No functional changes