Don't 404 extensions that could be handled by routing.

Fixes an error in #2750 where routed extensions would always return
404's for plugin requests. When a file extenion could be handled by
router, AssetDispatcher cannot 404 the request.

Refs #3305
Revert changed added in #2750.

While the had the potential to make 404s going through AssetDispatcher
much faster, they broke plugins + extension routing. While explicit
extensions could be fixed, routing all extensions could not. Because we
are trying to keep 2.x as API compatible as possible it makes sense to
revert the previous changes.
Merge pull request #3308 from cakephp/asset-dispatcher-fix

Don't 404 extensions that could be handled by routing.