simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumcloud/talk/get_participants/get_participants.dart

@@ -1,7 +1,6 @@
-import 'dart:convert';
-
 import 'package:http/http.dart' as http;
 
+import '../../nextcloud_ocs.dart';
 import '../talk_api.dart';
 import 'get_participants_response.dart';
 
@@ -10,12 +9,8 @@ class GetParticipants extends TalkApi<GetParticipantsResponse> {
   GetParticipants(this.token) : super('v4/room/$token/participants', null);
 
   @override
-  GetParticipantsResponse assemble(String raw) {
-    final decoded = jsonDecode(raw) as Map<String, dynamic>;
-    return GetParticipantsResponse.fromJson(
-      decoded['ocs'] as Map<String, dynamic>,
-    );
-  }
+  GetParticipantsResponse assemble(String raw) =>
+      GetParticipantsResponse.fromJson(NextcloudOcs.decode(raw));
 
   @override
   Future<http.Response> request(