removeTrack method
Implementation
Future<void> removeTrack(String id, String path) async {
final int index = _playlists.indexWhere((UPlaylist p) => p.id == id);
if (index < 0) return;
final List<String> next = List<String>.of(_playlists[index].paths)..remove(path);
_playlists[index] = _playlists[index].copyWith(paths: next);
await _persist();
}