Follow-up from "Prevent new note notifications from Ghost user"

The following discussion from !53699 (merged) should be addressed:

• @engwan started a discussion: (+1 comment)

  The NewIssueWorker also skips event creation when a user is already deleted, but I went with the more conservative change here and just suppressed notifications.

  Because I see we have https://gitlab.com/gitlab-org/gitlab/blob/6e27e2f65d10f242c0dcd6bf32e71e343e5161f4/ee/app/services/ee/notes/post_process_service.rb#L13 and I didn't want to change the behavior of that one.