Fixes for feature "Regroupment of center"

* allow more than 15 character in regroupment name
* remove unused methods in PickCenterType
* show only active Regroupment in form
* remove dead code and commented code

src/Bundle/ChillMainBundle/Resources/views/Export/new_centers_step.html.twig

@@ -44,16 +44,6 @@
             <h3 class="m-3">{{ 'Pick aggregated centers'|trans }}</h3>
             {{ form_widget(form.centers.regroupment) }}
 
-{#            {% if form.centers.children.g is defined %}#}
-{##}
-{#                <h3>{{ 'Pick aggregated centers'|trans }}</h3>#}
-{##}
-{#                {% for f in form.centers.children.g.children %}#}
-{#                    {{ form_row(f) }}#}
-{#                {% endfor %}#}
-{##}
-{#            {% endif %}#}
-
         </section>
 
         <p>{{ form_widget(form.submit, { 'attr' : { 'class' : 'btn btn-action btn-create' }, 'label' : 'Go to export options' } ) }}</p>