Remove clean_redis from cron.

It seems like this is no longer needed. Logs always show that 0 keys were cleaned.

May  5 16:40:05 mktadm1.ops.services.phx1.mozilla.com: [][] z.redis:DEBUG [0 0.0%] Dropping 0 keys. :/data/mkt.prod/www/marketplace.firefox.com/deploy-zamboni-prod-20140429221720-5f0e3c4fd7/zamboni/apps/amo/management/commands/clean_redis.py:87

@oremj r?