models: delete AdministrateursProcedure when destroying Administrateur

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/administrateur.rb

@@ -12,8 +12,7 @@ class Administrateur < ApplicationRecord
   include ActiveRecord::SecureToken
 
   has_and_belongs_to_many :instructeurs
-  has_many :administrateurs_procedures
-  has_many :procedures, through: :administrateurs_procedures
+  has_and_belongs_to_many :procedures
   has_many :services
 
   has_one :user, dependent: :nullify

spec/models/administrateur_spec.rb

@@ -3,7 +3,7 @@ describe Administrateur, type: :model do
 
   describe 'associations' do
     it { is_expected.to have_and_belong_to_many(:instructeurs) }
-    it { is_expected.to have_many(:procedures) }
+    it { is_expected.to have_and_belong_to_many(:procedures) }
   end
 
   describe "#renew_api_token" do