Remove dead dartdocs.dart file (#101215)

dev/devicelab/bin/tasks/dartdocs.dart

-// Copyright 2014 The Flutter Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-import 'package:flutter_devicelab/framework/framework.dart';
-import 'package:flutter_devicelab/framework/task_result.dart';
-
-Future<void> main() async {
-  await task(() async {
-    // TODO(jmagman): Remove once dartdocs builder can be deleted
-    // when https://github.com/flutter/flutter/issues/79798 rolls to stable.
-    return TaskResult.success(null);
-  });
-}