Remove auto_cors functionality

@nicotaing @rtjoseph11 @bgoldman @PikaDotus

Hey guys,

So I'm kind of torn on this. I just ran into an issue where this CORS implementation ran into some trouble, and on second thought, I'm not sure if this even belongs in Kenji in the first place. So instead of patching it to support PATCH; I figured I'd just remove it and recommend using [Rack::Cors][rack-cors] instead.

[rack-cors]: https://github.com/cyu/rack-cors

Thoughts?

-Kenneth