Remove unused alert on merge request compare

- originally added to show merge request compare errors
  (https://gitlab.com/gitlab-org/gitlab/-/commit/536373ad05b55c69442e7d7d6cb549791031cac2)
- removed in lieu for letting the form component handle the errors
  (https://gitlab.com/gitlab-org/gitlab/-/commit/c5c05f6a04ab3c791bc7c34dc74925731cf2ff94)
- No reference to the class .mr-compare-errors anywhere in the code

app/views/projects/merge_requests/creations/_new_compare.html.haml

@@ -4,7 +4,6 @@
 = form_for [@project, @merge_request], url: project_new_merge_request_path(@project), method: :get, html: { class: "merge-request-form js-requires-input" } do |f|
   - if params[:nav_source].present?
     = hidden_field_tag(:nav_source, params[:nav_source])
-  .hide.alert.alert-danger.mr-compare-errors
   .js-merge-request-new-compare.row{ 'data-source-branch-url': project_new_merge_request_branch_from_path(@source_project), 'data-target-branch-url': project_new_merge_request_branch_to_path(@source_project) }
     .col-lg-6
       .card.card-new-merge-request