models: delete AdministrateursInstructeur when destroying Instructeur

By default, `has_and_belongs_to_many` properly deletes the record in
the join table.

However, as the association is declared manually with a
`has_many / through`, it doesn't delete the joined record automatically.

As we also lack a foreign-key contraint on the join table, that means
a dangling record remains in the join table.

To fix this, let's declare it a proper `has_and_belongs_to_many`
association, which will let the join record be deleted automatically
on destroy.

app/models/instructeur.rb

@@ -11,8 +11,7 @@
 #  agent_connect_id         :string
 #
 class Instructeur < ApplicationRecord
-  has_many :administrateurs_instructeurs
+  has_and_belongs_to_many :administrateurs
-  has_many :administrateurs, through: :administrateurs_instructeurs
 
   has_many :assign_to, dependent: :destroy
   has_many :groupe_instructeurs, through: :assign_to

spec/models/instructeur_spec.rb

@@ -12,6 +12,10 @@ describe Instructeur, type: :model do
     procedure_3
   end
 
+  describe 'associations' do
+    it { is_expected.to have_and_belong_to_many(:administrateurs) }
+  end
+
   describe 'follow' do
     let(:dossier) { create :dossier }
     let(:already_followed_dossier) { create :dossier }