Combine `flutter_tool_startup` devicelab tests (#125226)

`flutter_tool_startup__windows/linux/macos` were identical. Use the same task for all three.

.ci.yaml

@@ -4842,7 +4842,7 @@ targets:
     properties:
       tags: >
         ["devicelab", "hostonly", "windows"]
-      task_name: flutter_tool_startup__windows
+      task_name: flutter_tool_startup
 
   - name: Linux flutter_tool_startup__linux
     recipe: devicelab/devicelab_drone
@@ -4851,14 +4851,14 @@ targets:
     properties:
       tags: >
         ["devicelab", "hostonly", "linux"]
-      task_name: flutter_tool_startup__linux
+      task_name: flutter_tool_startup
 
   - name: Mac_benchmark flutter_tool_startup__macos
     presubmit: false
     recipe: devicelab/devicelab_drone
     timeout: 60
     properties:
-      task_name: flutter_tool_startup__macos
+      task_name: flutter_tool_startup
 
   - name: Linux flutter_packaging
     recipe: packaging/packaging

dev/devicelab/bin/tasks/flutter_tool_startup__macos.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/tasks/flutter_tool_startup.dart';
-
-Future<void> main() async {
-  await task(flutterToolStartupBenchmarkTask);
-}

dev/devicelab/bin/tasks/flutter_tool_startup__windows.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/tasks/flutter_tool_startup.dart';
-
-Future<void> main() async {
-  await task(flutterToolStartupBenchmarkTask);
-}