simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumcloud/talk/room/get_room.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_room_params.dart';
 import 'get_room_response.dart';
@@ -11,10 +10,8 @@ class GetRoom extends TalkApi<GetRoomResponse> {
   GetRoom(this.params) : super('v4/room', null, getParameters: params.toJson());
 
   @override
-  GetRoomResponse assemble(String raw) {
-    final decoded = jsonDecode(raw) as Map<String, dynamic>;
-    return GetRoomResponse.fromJson(decoded['ocs'] as Map<String, dynamic>);
-  }
+  GetRoomResponse assemble(String raw) =>
+      GetRoomResponse.fromJson(NextcloudOcs.decode(raw));
 
   @override
   Future<http.Response> request(