Remove redeemable code

I don't think this feature it was ever used. It was introduced in commit
49dec6061 as part of a feature that was removed in commits 1cd47da9d and
c45a0bd8ac.

app/controllers/users/registrations_controller.rb

@@ -6,9 +6,7 @@ class Users::RegistrationsController < Devise::RegistrationsController
   invisible_captcha only: [:create], honeypot: :address, scope: :user
 
   def new
-    super do |user|
-      user.use_redeemable_code = true if params[:use_redeemable_code].present?
-    end
+    super
   end
 
   def create
@@ -64,18 +62,13 @@ class Users::RegistrationsController < Devise::RegistrationsController
   private
 
     def sign_up_params
-      if params[:user].present? && params[:user][:redeemable_code].blank?
-        params[:user].delete(:redeemable_code)
-      end
-
       params.require(:user).permit(allowed_params)
     end
 
     def allowed_params
       [
         :username, :email, :password,
-        :password_confirmation, :terms_of_service, :locale,
-        :redeemable_code
+        :password_confirmation, :terms_of_service, :locale
       ]
     end