[HttpKernel] Revert BC break introduced by #8957

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

464439d195fc895079176498cb28d182dc8aef14 (https://github.com/symfony/symfony/pull/8957) creates a BC break. At least the SonataPage CMS used with the "host and path" strategy (http://sonata-project.org/bundles/page/master/doc/reference/multisite.html#host-and-path-strategy) is not working anymore because of this patch.

SonataPage extends the `Request` class without using the new factory system (there is probably a cleaner way to implement the "host and path" strategy in the CMS too).

When a subrequest is handled by `InlineFragment`, a request of type `Symfony\Component\HttpFoundation\Request` is instantiated instead of the expected `Sonata\PageBundle\Request\SiteRequest`. Then, SonataPage fail processing the subrequest.

I've just reverted the part of this commit changing the previous behavior, the factory system is still working.

ping @fabpot @rande @Sinjonathan