Consider removing dead code in app/presenters/ci/pipeline_presenter.rb

The following discussion from !133509 (merged) should be addressed:

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

  https://gitlab.com/gitlab-org/gitlab/-/jobs/5289013474 is failing because these lines aren't actually being tested. Do we know if that's just a false reading?

all_related_merge_request_links appears to be dead code. It seems like it used to be used by all_related_merge_request_text, but b481c54c might have dropped the use of this.

/cc: @pburdette