RFC - Remove CacheHelper and related features.

CacheHelper is going to be split into a standalone plugin for 3.x. Removing this code will reduce confusion in the future about how to do caching.

I had originally planned to [replace CacheHelper with an ESI based solution](https://github.com/markstory/cakephp/wiki/CacheHelper-replacement). However, after discussing it with other people on the core team further, we came to the conclusion that this was a bad idea :tm:. It was a bad idea for a few reasons:

1. We'd be implementing a sub-par version of [varnish](https://www.varnish-cache.org/) at best. At worst it would be a buggy difficult to maintain burden.
2. The behavior of our reverse proxy and varnish would be inconsistent. An in process reverse proxy would share memory with the master request and possibly suffer from global state issues, which would change should an application be deployed behind varnish in production.
3. The time spent building this feature could be better used to develop more important features like migrations, debug kit or fix bugs in CakePHP itself.

### TODO

* [ ] Update app repo.
* [ ] Update docs.