Remove redundant references to trusting HttpCache

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | 

Removes references to making `HttpCache` a trusted proxy as https://github.com/symfony/symfony/pull/11937 would cause it to be handled automatically.

Alternatively it could be reworded, but I'm not sure it's useful to say that this happens (since it was a bit odd to have to configure Symfony to let one part of the code base trust another).