ref(controller): remove redis cache library and settings

`CACHE_URL` and the python Redis library were not actually being used by the controller. We didn't remove Redis completely in #1834 because I thought it would be trivial to connect. Apparently it requires additional Django middleware and configuration to use Redis for caching. While I have seen session and object caching speed up Django deployments before, this is not a bottleneck for the Deis controller, and simpler == better.