Remove sidebar_refactor ref in startup_css scripts

- Since the sidebar_refactor ff doesn't exist anymore
  we can simply remove our reference to it and the
  reference to the removed fixture.

scripts/frontend/startup_css/constants.js

@@ -49,7 +49,6 @@ const createMainOutput = ({ outFile, cssKeys, type }) => ({
   outFile,
   htmlPaths: [
     path.join(FIXTURES_ROOT, `startup_css/project-${type}.html`),
-    path.join(FIXTURES_ROOT, `startup_css/project-${type}-legacy-sidebar.html`),
     path.join(FIXTURES_ROOT, `startup_css/project-${type}-signed-out.html`),
   ],
   cssKeys,

spec/frontend/fixtures/startup_css.rb

@@ -10,7 +10,6 @@ RSpec.describe 'Startup CSS fixtures', type: :controller do
   render_views
 
   before(:all) do
-    stub_feature_flags(sidebar_refactor: true)
     clean_frontend_fixtures('startup_css/')
   end
 
@@ -31,17 +30,6 @@ RSpec.describe 'Startup CSS fixtures', type: :controller do
       expect(response).to be_successful
     end
 
-    it "startup_css/project-#{type}-legacy-sidebar.html" do
-      stub_feature_flags(sidebar_refactor: false)
-
-      get :show, params: {
-        namespace_id: project.namespace.to_param,
-        id: project
-      }
-
-      expect(response).to be_successful
-    end
-
     it "startup_css/project-#{type}-signed-out.html" do
       sign_out(user)