Remove ununsed CONTRIBUTING link on edit MR form.

The code is never reached because the new and edit MR forms have diverged,
and the CONTRIBUTING link is not shown when editing MRs.

app/views/projects/merge_requests/_form.html.haml

 = form_for [@project, @merge_request], html: { class: "merge-request-form form-horizontal" } do |f|
-  .row
-    .col-sm-2
-    .col-sm-10
-      - if @repository.contribution_guide && !@merge_request.persisted?
-        - contribution_guide_url = project_blob_path(@project, tree_join(@repository.root_ref, @repository.contribution_guide.name))
-        .alert.alert-info
-          Please review the
-          %strong #{link_to "guidelines for contribution", contribution_guide_url}
-          to this repository.
-
   .merge-request-form-info
     = render 'projects/issuable_form', f: f, issuable: @merge_request
     .form-group