TF-3157 Remove unnecessary background service worker

lib/features/push_notification/data/datasource/web_socket_datasource.dart

@@ -1,6 +1,9 @@
 import 'package:jmap_dart_client/jmap/account_id.dart';
 import 'package:jmap_dart_client/jmap/core/session/session.dart';
+import 'package:web_socket_channel/web_socket_channel.dart';
 
 abstract class WebSocketDatasource {
-  Stream<dynamic> getWebSocketChannel(Session session, AccountId accountId);
+  Future<WebSocketChannel> getWebSocketChannel(
+    Session session,
+    AccountId accountId);
 }