Remove unused member

src/Audio/soundTrack.ts

@@ -35,7 +35,6 @@ export class SoundTrack {
 
     private _outputAudioNode: Nullable<GainNode>;
     private _scene: Scene;
-    private _isMainTrack: boolean = false;
     private _connectedAnalyser: Analyser;
     private _options: ISoundTrackOptions;
     private _isInitialized = false;
@@ -64,7 +63,6 @@ export class SoundTrack {
 
             if (this._options) {
                 if (this._options.volume) { this._outputAudioNode.gain.value = this._options.volume; }
-                if (this._options.mainTrack) { this._isMainTrack = this._options.mainTrack; }
             }
 
             this._isInitialized = true;