Code review comment for lp:~stub/launchpad/bug-490239-session-tuning

Currently, our session machinery prunes old session information during a request. This is silly because it might take a while, and under high load, might contribute to a death spiral of the production systems and deletion requests time out and are retried by subsequent requests. Although this is still just a theory, removing this code still seems a good idea and batch processes to prune the old data are already in place.