removed obsolete cookbook entry

doc/cookbook/translating_validation_messages.rst

-Translating Validation Messages
-===============================
-
-When working with Symfony validator, a common task would be to show localized
-validation messages.
-
-In order to do that, you will need to register translator and point to
-translated resources::
-
-    $app->register(new Silex\Provider\TranslationServiceProvider(), array(
-        'locale' => 'sr_Latn',
-        'translator.domains' => array(),
-    ));
-
-    $app->before(function () use ($app) {
-        $app['translator']->addLoader('xlf', new Symfony\Component\Translation\Loader\XliffFileLoader());
-        $app['translator']->addResource('xlf', __DIR__.'/vendor/symfony/validator/Symfony/Component/Validator/Resources/translations/validators/validators.sr_Latn.xlf', 'sr_Latn', 'validators');
-    });
-
-And that's all you need to load translations from Symfony ``xlf`` files.