user_groups: Remove unused html_selector passed to ListWidget.create.

html_selector is only used when rendering a single item in the list
using insert_rendered_row and render_item, which are not used for
group members list in group creation form.

web/src/user_group_create_members.ts

@@ -145,9 +145,6 @@ export function build_widgets(): void {
             },
         },
         $simplebar_container,
-        html_selector(user) {
-            return $(`#${CSS.escape("user_checkbox_" + user.user_id)}`);
-        },
     });
     pill_widget.appendValue(current_user.email);
 }