Remove the mandataire_social guess

Rationale:
- is true for only 0.06% of Dossiers
- is not displayed anymore anyways
- will be a rare case for now
  * FC adoption is not insane
  * FC used more for personal than professional
    matters
  * not always the mandataires sociaux that fill
    dossiers for their companies, so not that
    relevant
- we can add it back later

app/serializers/dossier_serializer.rb

@@ -4,7 +4,6 @@ class DossierSerializer < ActiveModel::Serializer
     :updated_at,
     :archived,
     :email,
-    :mandataire_social,
     :state,
     :simplified_state,
     :initiated_at,