fix(champ): rewire champ.mandatory? to type_de_champ.mandatory?

it fixes behaviour in drop_down_list_component which add option if not mandatory (!= not required)

app/models/champ.rb

@@ -60,6 +60,7 @@ class Champ < ApplicationRecord
     :rna?,
     :siret?,
     :stable_id,
+    :mandatory?,
     to: :type_de_champ
 
   scope :updated_since?, -> (date) { where('champs.updated_at > ?', date) }
@@ -93,10 +94,6 @@ class Champ < ApplicationRecord
     @sections ||= dossier.sections_for(self)
   end
 
-  def mandatory?
-    type_de_champ.mandatory? && visible?
-  end
-
   # used for the `required` html attribute
   # check visibility to avoid hidden required input
   # which prevent the form from being sent.