Remove unused subgroups route for groups in EE

This was removed in:
https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/14055

We then added it back in the following CE -> EE merge, but I think that
we only needed some other routes, not this one:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/3308#note_46525961

ee/app/controllers/ee/groups_controller.rb

@@ -15,7 +15,7 @@ module EE
         push_frontend_feature_flag(:scoped_labels, @group, type: :licensed)
       end
 
-      feature_category :subgroups, [:restore, :subgroups]
+      feature_category :subgroups, [:restore]
     end
 
     override :render_show_html

ee/config/routes/group.rb

 # frozen_string_literal: true
 
 constraints(::Constraints::GroupUrlConstrainer.new) do
-  scope(path: 'groups/*id',
-        controller: :groups,
-        constraints: { id: Gitlab::PathRegex.full_namespace_route_regex, format: /(html|json|atom|ics)/ }) do
-    scope(path: '-') do
-      get :subgroups, as: :subgroups_group # rubocop:todo Cop/PutGroupRoutesUnderScope
-    end
-  end
-
   scope(path: 'groups/*group_id/-',
         module: :groups,
         as: :group,