DEV: removes default service actions (#26078)

Previously services would let you define a high level default `def default_actions_for_service; end` which would define various handlers like `on_success`, after months of usage we consider the cons are superior to the pros here.

Two mains cons:
- people would often not understand where the handling was coming from
- it's easy to miss a case when you write your specs

plugins/chat/app/controllers/chat/api_controller.rb

@@ -3,18 +3,5 @@
 module Chat
   class ApiController < ::Chat::BaseController
     include Chat::WithServiceHelper
-
-    private
-
-    def default_actions_for_service
-      proc do
-        on_success { render(json: success_json) }
-        on_failure { render(json: failed_json, status: 422) }
-        on_failed_policy(:invalid_access) { raise Discourse::InvalidAccess }
-        on_failed_contract do |contract|
-          render(json: failed_json.merge(errors: contract.errors.full_messages), status: 400)
-        end
-      end
-    end
   end
 end