Clean up examples directory

sky/examples/painting/circle.sky

-<sky>
-<style>
-div {
-    height: 200px;
-    background-color: lightblue;
-}
-</style>
-<div id="canvas" />
-<script>
-import 'dart:math' as math;
-import 'dart:sky';
-
-void main() {
-  var element = document.getElementById('canvas');
-  element.requestPaint((PaintingContext context) {
-    Paint paint = new Paint();
-    paint.setARGB(128, 0, 255, 0);
-    double radius = math.min(context.width, context.height) / 2.0;
-    context.drawCircle(context.width / 2.0, context.height / 2.0, radius, paint);
-    context.commit();
-  });
-}
-</script>
-</sky>