Remove deprecated `/admin/application_settings` redirect

12.8 deprecated the endpoint and redirected to
`/admin/application_settings/general`. For 13.0 the endpoint can
be removed since it's potentially a breaking change
for a user that had the endpoint bookmarked.

changelogs/unreleased/dblessing-remove-app-settings-redirect.yml

+---
+title: Remove deprecated /admin/application_settings redirect
+merge_request: 30532
+author:
+type: removed

config/routes/admin.rb

@@ -116,10 +116,6 @@ namespace :admin do
   end
 
   resource :application_settings, only: :update do
-    # This redirect should be removed with 13.0 release.
-    # https://gitlab.com/gitlab-org/gitlab/issues/199427
-    get '/', to: redirect('admin/application_settings/general'), as: nil
-
     resources :services, only: [:index, :edit, :update]
     resources :integrations, only: [:edit, :update] do
       member do