Remove unused cache

lib/utils/file_util.dart

@@ -11,7 +11,6 @@ import 'package:flutter_cache_manager/flutter_cache_manager.dart';
 import 'package:flutter_image_compress/flutter_image_compress.dart';
 import 'package:photos/core/cache/image_cache.dart';
 import 'package:photos/core/cache/thumbnail_cache.dart';
-import 'package:photos/core/cache/thumbnail_cache_manager.dart';
 import 'package:photos/core/cache/video_cache_manager.dart';
 import 'package:photos/core/configuration.dart';
 import 'package:photos/core/constants.dart';