fix sf4 deprecated: remove choices_as_values form option

Form/UserType.php

@@ -63,7 +63,6 @@ class UserType extends AbstractType
                         ),
                         'expanded' => false,
                         'multiple' => false,
-                        'choices_as_values' => true // Can be removed when upgraded to Sf3.
                         ))
                 );
         }