Remove deprecation warning during cron job.

Change crontab entry to use clearsessions in favour of cleanup as cleanup has
been deprecated.

Testing Done:
Running `./reviewboard/manage.py cleanup` causes warning messages to be
outputted, but running `./reviewboard/manage.py clearsessions` does not.

Bugs closed: 3535

Reviewed at https://reviews.reviewboard.org/r/6300/