Remove unused "error" variable
Remove unused "error" variable

```
.pub-cache/hosted/pub.dartlang.org/assets_audio_player-3.0.3+2/darwin/Classes/Music.swift:612:25: Initialization of variable 'error' was never used; consider replacing with assignment to '_' or removing it
```
florent37/Flutter-AssetsAudioPlayer,florent37/Flutter-AssetsAudioPlayer,florent37/Flutter-AssetsAudioPlayer,florent37/Flutter-AssetsAudioPlayer,florent37/Flutter-AssetsAudioPlayer,florent37/Flutter-AssetsAudioPlayer