Removed 'nav nav-tabs' class

At the moment, ``Admin`` forces to use ``nav navbar-nav``:
```$menu->setChildrenAttribute('class', 'nav navbar-nav');```

I think this should be delegated to the corresponding template. When you want to change the way the menu is rendered, you should only need to change the template.

Example: I want to render this menu as a vertical menu (like before).  I use Braincrafted Bootstrap and have in my template:
```
{% block side_menu %}{{ knp_menu_render(admin.sidemenu(action), {'currentClass' : 'active', 'style': 'list'}) }}{% endblock %}
```
Now this is rendered as: ``<ul class="nav navbar-nav nav-list nav"></ul>``. How to remove ``nav navbar-nav``?