Skip generation of default Devise routes for admin

This commit updates the backend route configuration for Devise, to skip
generating the default route set. We already specify these routes with
much more convenient names (`/admin/login` vs `/user/spree_user/sign_in`).

config/routes.rb

@@ -47,13 +47,12 @@
       devise_for(:spree_user, {
         class_name: 'Spree::User',
         singular: :spree_user,
+        skip: :all,
+        path_names: { sign_out: 'logout' },
         controllers: {
           sessions: 'spree/admin/user_sessions',
           passwords: 'spree/admin/user_passwords'
         },
-        skip: [:unlocks, :omniauth_callbacks, :registrations],
-        path_names: { sign_out: 'logout' },
-        path_prefix: :user
       })
 
       devise_scope :spree_user do