views: remove "Select a value" text on radio lists

This text:

- Isn't present on other form controls
- Should only be displayed if the field is mandatory anyway

app/views/shared/dossiers/editable_champs/_drop_down_list.html.haml

@@ -1,8 +1,6 @@
 - if champ.options?
   - if champ.render_as_radios?
     %fieldset.radios
-      %legend.mandatory-explanation
-        Sélectionnez une des valeurs
       - champ.enabled_non_empty_options.each do |option|
         %label
           = form.radio_button :value, option