Remove EE-specific code from 'Discussion' models

This does not do Issue, as that's being handled in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8047.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/6679.

ee/app/models/ee/milestone.rb

@@ -3,9 +3,10 @@
 module EE
   module Milestone
     extend ActiveSupport::Concern
-    include Elastic::MilestonesSearch
 
     prepended do
+      include Elastic::MilestonesSearch
+
       has_many :boards
     end