Always construct ajax url based on resource collection path

No need to fall back to constructing it via `polymorphic_path` since
if we cannot fine the admin resource to get `route_collection_path`,
we will also not be able to use to determine `ajax_resource_class`.

Author: tf

lib/activeadmin/searchable_select/select_input_extension.rb

@@ -45,19 +45,11 @@ def collection_from_options
 
       def ajax_url
         return unless options[:ajax]
-        template.polymorphic_path([active_admin_prefix, resource_route_name || ajax_resource_class],
-                                  action: option_collection.collection_action_name,
-                                  **ajax_params)
-      end
-
-      def active_admin_prefix
-        template.active_admin_namespace.route_prefix
-      end
-
-      def resource_route_name
-        active_admin_namespace = template.active_admin_namespace
-        str = active_admin_namespace.resource_for(ajax_resource_class)&.route_collection_path
-        str&.split('/')&.last
+        [ajax_resource.route_collection_path,
+         '/',
+         option_collection.collection_action_name,
+         '?',
+         ajax_params.to_query].join
       end
 
       def all_options_collection