Merge pull request #7803 from cirosantilli/rm-unused-mr-contributing

Remove ununsed CONTRIBUTING link on edit MR form.

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