ffmpegdecoder: fixed double mutex lock

Error() would call Close() getting stuck in a double mutex lock that could
stall workers. This commit fixes that.

app/task/index/index.cpp

@@ -18,11 +18,6 @@ void IndexTask::Action()
 
     decoder->set_stream(stream_);
 
-    // Force multithreading for faster indexing
-    if (decoder->id() == "ffmpeg") {
-      static_cast<FFmpegDecoder*>(decoder.get())->SetMultithreading(true);
-    }
-
     connect(decoder.get(), &Decoder::IndexProgress, this, &IndexTask::ProgressChanged);
 
     decoder->Open();