delete the doc for a number of cache provider methods that don't exist

[chuanma]

http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/caching.html#deleting

http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/caching.html#counting

I checked source code. Most of the methods don't exist. They are better to be removed from the doc. I tried to use those functions and was surprised that didn't exist.

I'll send a PR soon.

[chuanma]

Seems this repo is not maintained. I'll send a PR to the new repo.