Revert "merged branch ruimarinho/http_digest (PR #3778)"

This reverts commit eb6a26f57269d844cf9ef90150e3e0925d5e5bf3, reversing
changes made to a10fee16c193d93080c22528e58b1fd018341229.

src/Symfony/Bundle/SecurityBundle/DependencyInjection/Security/Factory/HttpDigestFactory.php

@@ -62,7 +62,7 @@ class HttpDigestFactory implements SecurityFactoryInterface
             ->children()
                 ->scalarNode('provider')->end()
                 ->scalarNode('realm')->defaultValue('Secured Area')->end()
-                ->scalarNode('key')->isRequired()->cannotBeEmpty()->end()
+                ->scalarNode('key')->cannotBeEmpty()->end()
             ->end()
         ;
     }