cleanup eslint warning/ weird statement

src/animation/AnimationMixer.js

@@ -428,9 +428,7 @@ AnimationMixer.prototype = Object.assign( Object.create( EventDispatcher.prototy
 
 		delete bindingByName[ trackName ];
 
-		remove_empty_map: {
-
-			for ( var _ in bindingByName ) break remove_empty_map; // eslint-disable-line no-unused-vars
+		if ( Object.keys( bindingByName ).length === 0 ) {
 
 			delete bindingsByRoot[ rootUuid ];