Stop exposing MR refs in favor of persistent pipeline refs

Summary

We've added a persisted ref for pipeline at !17043 (merged).

With this change, we don't need to expose merge request refs to runners anymore.

• refs/merge-requests/:iid/head
• refs/merge-requests/:iid/merge
• refs/merge-requests/:iid/train

This introduces a good effect that pipelines can keep running even if the ref is removed by the system, which currently fails by failed to find ref (e.g. When a MR is dropped from a merge train)

Proposal

diff --git a/app/presenters/ci/build_runner_presenter.rb b/app/presenters/ci/build_runner_presenter.rb
index 8e469795581..f03c69e0031 100644
--- a/app/presenters/ci/build_runner_presenter.rb
+++ b/app/presenters/ci/build_runner_presenter.rb
@@ -34,7 +34,6 @@ module Ci
 
     def refspecs
       specs = []
-      specs << refspec_for_pipeline_ref if merge_request_ref?
       specs << refspec_for_persistent_ref if persistent_ref_exist?
 
       if git_depth > 0
@@ -87,10 +86,6 @@ module Ci
       "+#{Gitlab::Git::TAG_REF_PREFIX}#{ref}:#{RUNNER_REMOTE_TAG_PREFIX}#{ref}"
     end
 
-    def refspec_for_pipeline_ref
-      "+#{ref}:#{ref}"
-    end
-
     def refspec_for_persistent_ref
       "+#{persistent_ref_path}:#{persistent_ref_path}"
     end