update docs to show ImageBitmapLoader does not support progress events

docs/api/loaders/ImageBitmapLoader.html

@@ -36,10 +36,8 @@
 				var material = new THREE.MeshBasicMaterial( { map: texture } );
 			},
 
-			// onProgress callback
-			function ( xhr ) {
-				console.log( (xhr.loaded / xhr.total * 100) + '% loaded' );
-			},
+			// onProgress callback currently not supported
+			undefined,
 
 			// onError callback
 			function ( err ) {