remove useless use of variable

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?    | no
| Applies to    | all
| Fixed tickets | 

Removed the use of the ``$user`` variable in the [Dynamic generation for submitted Forms](http://symfony.com/doc/current/cookbook/form/dynamic_form_modification.html#dynamic-generation-for-submitted-forms) example. It was probably a leftover from the [Customizing the Form Type](http://symfony.com/doc/current/cookbook/form/dynamic_form_modification.html#customizing-the-form-type) section.