remove dead code - 'collapsed_nav' cookie no longer used since a1fbdbb6 (see MR !4579)

app/helpers/nav_helper.rb

 module NavHelper
-  def nav_menu_collapsed?
-    cookies[:collapsed_nav] == 'true'
-  end
-
-  def nav_sidebar_class
-    if nav_menu_collapsed?
-      "sidebar-collapsed"
-    else
-      "sidebar-expanded"
-    end
-  end
-
   def page_sidebar_class
     if pinned_nav?
       "page-sidebar-expanded page-sidebar-pinned"
@@ -24,7 +12,6 @@ module NavHelper
       current_path?('merge_requests#builds') ||
       current_path?('merge_requests#conflicts') ||
       current_path?('merge_requests#pipelines') ||
-      
       current_path?('issues#show')
       if cookies[:collapsed_gutter] == 'true'
         "page-gutter right-sidebar-collapsed"

app/views/layouts/_page.html.haml

 .page-with-sidebar{ class: "#{page_sidebar_class} #{page_gutter_class}" }
-  .sidebar-wrapper.nicescroll{ class: nav_sidebar_class }
+  .sidebar-wrapper.nicescroll
     .sidebar-action-buttons
       = link_to '#', class: 'nav-header-btn toggle-nav-collapse', title: "Open/Close" do
         %span.sr-only Toggle navigation

spec/helpers/nav_helper_spec.rb

-require 'spec_helper'
-
-# Specs in this file have access to a helper object that includes
-# the NavHelper. For example:
-#
-# describe NavHelper do
-#   describe "string concat" do
-#     it "concats two strings with spaces" do
-#       expect(helper.concat_strings("this","that")).to eq("this that")
-#     end
-#   end
-# end
-describe NavHelper do
-  describe '#nav_menu_collapsed?' do
-    it 'returns true when the nav is collapsed in the cookie' do
-      helper.request.cookies[:collapsed_nav] = 'true'
-      expect(helper.nav_menu_collapsed?).to eq true
-    end
-
-    it 'returns false when the nav is not collapsed in the cookie' do
-      helper.request.cookies[:collapsed_nav] = 'false'
-      expect(helper.nav_menu_collapsed?).to eq false
-    end
-  end
-end