Revert "Material: Deprecated skinning. See #10754."

This reverts commit 118fedf.

Author: mrdoob

src/Three.Legacy.js

@@ -994,20 +994,6 @@ Object.defineProperties( Uniform.prototype, {
 
 Object.defineProperties( Material.prototype, {
 
-	skinning: {
-		get: function () {
-
-			console.warn( 'THREE.Material: .skinning has been removed.' );
-
-		},
-		set: function () {
-
-			console.warn( 'THREE.Material: .skinning has been removed.' );
-
-		}
-
-	},
-
 	wrapAround: {
 		get: function () {