Follow-up from "Prevent closing of merged merge requests"

The following discussion from !208810 (merged) should be addressed:

• @dstull started a discussion: (+5 comments)

  question:

  in app/services/issuable_base_service.rb version of this there is a case where it seems like it is trying to cover the idea of there being an else condition and the service class not existing.

  See here

  Is there a need for this here, or is that invalid for our case here?