Removed: unneeded EzPublishCache intermediate class

To be evaluated together with: https://github.com/ezsystems/ezpublish-kernel/pull/360

New configuration to document:

```Apache
    # Customize the Http cache class to use when USE_HTTP_CACHE is set to "1"
    # Defaults to "eZ\Bundle\EzPublishCoreBundle\HttpCache" if omitted
    #SetEnv HTTP_CACHE_CLASS eZ\Bundle\EzPublishCoreBundle\HttpCache
```