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/system_note_metadata.rb

@@ -11,7 +11,6 @@ class SystemNoteMetadata < ActiveRecord::Base
   TYPES_WITH_CROSS_REFERENCES = %w[
     commit cross_reference
     close duplicate
-    relate unrelate
     moved
   ].freeze