[url_launcher] Remove unused dartHostTestHandler (#10324)

`url_launcher_windows` was setting a `dartHostTestHandler` in its Pigeon config file, but it was not actually using the test generator.

Part of flutter/flutter#159886

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Author: stuartmorgan-g

packages/url_launcher/url_launcher_windows/pigeons/messages.dart

@@ -13,7 +13,7 @@ import 'package:pigeon/pigeon.dart';
     copyrightHeader: 'pigeons/copyright.txt',
   ),
 )
-@HostApi(dartHostTestHandler: 'TestUrlLauncherApi')
+@HostApi()
 abstract class UrlLauncherApi {
   bool canLaunchUrl(String url);
   bool launchUrl(String url);