[HttpFoundation] remove unnecessary code in Response::prepare()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

The code piece just doesn't make sense. Maybe the idea was to get `Content-Length` from `Request`?