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.

app/models/legacy_diff_note.rb

@@ -7,11 +7,7 @@
 #
 # A note of this type is never resolvable.
 class LegacyDiffNote < Note
-  # Elastic search configuration (it does not support STI properly)
-  document_type 'doc'
-  index_name [Rails.application.class.parent_name.downcase, Rails.env].join('-')
-  include Elastic::NotesSearch
-
+  prepend EE::LegacyDiffNote
   include NoteOnDiff
 
   serialize :st_diff # rubocop:disable Cop/ActiveRecordSerialize