Remove `skin` property from `Bone` (#9951)

src/objects/Bone.js

@@ -6,14 +6,12 @@ import { Object3D } from '../core/Object3D';
  * @author ikerr / http://verold.com
  */
 
-function Bone( skin ) {
+function Bone() {
 
 	Object3D.call( this );
 
 	this.type = 'Bone';
 
-	this.skin = skin;
-
 }
 
 Bone.prototype = Object.assign( Object.create( Object3D.prototype ), {
@@ -26,8 +24,6 @@ Bone.prototype = Object.assign( Object.create( Object3D.prototype ), {
 
 		Object3D.prototype.copy.call( this, source );
 
-		this.skin = source.skin;
-
 		return this;
 
 	}

src/objects/SkinnedMesh.js

@@ -35,7 +35,7 @@ function SkinnedMesh( geometry, material, useVertexTexture ) {
 
 			gbone = this.geometry.bones[ b ];
 
-			bone = new Bone( this );
+			bone = new Bone();
 			bones.push( bone );
 
 			bone.name = gbone.name;