[web] fix clipboard.getData (#17544)

* remove the fake reply for paste message. the clipboard.dart will reply with the content of the clipboard

* also remove the fake answer for copy

lib/web_ui/lib/src/engine/window.dart

@@ -392,11 +392,9 @@ class EngineWindow extends ui.Window {
             return;
           case 'Clipboard.setData':
             ClipboardMessageHandler().setDataMethodCall(decoded, callback);
-            _replyToPlatformMessage(callback, codec.encodeSuccessEnvelope(true));
             return;
           case 'Clipboard.getData':
             ClipboardMessageHandler().getDataMethodCall(callback);
-            _replyToPlatformMessage(callback, codec.encodeSuccessEnvelope(true));
             return;
         }
         break;