Remove unused method on subscription controller

This method was added in
https://gitlab.com/gitlab-org/gitlab/-/commit/05b4523598cbe9aa499ef3e47eecf19f544b80eb
and stopped being called after changes in
https://gitlab.com/gitlab-org/gitlab/-/commit/35f411479f43ac9cd77560f46784e4f0f5c88163.

Remove the method as it is no longer being used.

ee/app/controllers/subscriptions_controller.rb

@@ -108,10 +108,6 @@ class SubscriptionsController < ApplicationController
     Gitlab::SubscriptionPortal::Client
   end
 
-  def customer_portal_new_subscription_url
-    "#{EE::SUBSCRIPTIONS_URL}/subscriptions/new?plan_id=#{params[:plan_id]}&transaction=create_subscription"
-  end
-
   def redirect_unauthenticated_user(from = action_name)
     return if current_user