Remove non-functional sub-menu from "Merge Requests"

At the moment, we have a non-functional markup generated for the sub-menu of "Merge Requests" navigation item in the left0-hand project sidebar:

          %ul.sidebar-sub-level-items.is-fly-out-only
            = nav_link(controller: :merge_requests, html_options: { class: "fly-out-top-item" } ) do
              = link_to project_merge_requests_path(@project) do
                %strong.fly-out-top-item-name
                  = _('Merge requests')
                %span.badge.badge-pill.count.merge_counter.js-merge-counter.fly-out-badge
                  = number_with_delimiter(@project.open_merge_requests_count)

At the moment, the "Merge Requests" navigation item in the left-hand sidebar is used as a standalone item without the sub-items, hence it makes sense to get rid of the excessive markup in the course of the bigger effort of refactoring the sidebar of the "Create:Editor -- 13.11 - 14.0 -- Update Left Sidebar" epic.