DEV: Drop WithServiceHelper

This patch removes the `with_service` helper from the code base.
Instead, we can pass a block with actions directly to the `.call` method
of a service.

This simplifies how to use services:
- use `.call` without a block to run the service and get its result
  object.
- use `.call` with a block of actions to run the service and execute
  arbitrary code depending on the service outcome.

It also means a service is now “self-contained” and can be used anywhere
without having to include a helper or whatever.

app/controllers/admin/admin_controller.rb

@@ -1,8 +1,6 @@
 # frozen_string_literal: true
 
 class Admin::AdminController < ApplicationController
-  include WithServiceHelper
-
   requires_login
   before_action :ensure_admin