update document on Doctrine cache provider

A number of methods have been deleted long time ago. But they still show up on the document page.

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 they didn't exist.