Remove Sky tests that we don't intend to port to the new world

sky/tests/custom-elements/callbacks.sky

-<sky>
-<script>
-import "../resources/third_party/unittest/unittest.dart";
-import "../resources/unit.dart";
-
-import "dart:sky";
-
-class FooElement extends Element {
-  final String tagName = "foo";
-
-  attachedCallback() {
-    ++attachedCallbackCount;
-  }
-
-  detachedCallback() {
-    ++detachedCallbackCount;
-  }
-
-  attributeChangedCallback(String name, String oldValue, String newValue) {
-    ++attributeChangedCallbackCount;
-  }
-
-  int attachedCallbackCount = 0;
-  int detachedCallbackCount = 0;
-  int attributeChangedCallbackCount = 0;
-}
-
-
-void main() {
-  initUnit();
-
-  document.registerElement("foo", FooElement);
-
-  test("callbacks should be called", () {
-    Element sky = document.querySelector("sky");
-    FooElement foo = document.createElement("foo");
-    expect(foo.attachedCallbackCount, equals(0));
-    expect(foo.detachedCallbackCount, equals(0));
-    expect(foo.attributeChangedCallbackCount, equals(0));
-    sky.appendChild(foo);
-    expect(foo.attachedCallbackCount, equals(1));
-    expect(foo.detachedCallbackCount, equals(0));
-    expect(foo.attributeChangedCallbackCount, equals(0));
-    foo.setAttribute("bar", "baz");
-    expect(foo.attachedCallbackCount, equals(1));
-    expect(foo.detachedCallbackCount, equals(0));
-    expect(foo.attributeChangedCallbackCount, equals(1));
-    foo.remove();
-    expect(foo.attachedCallbackCount, equals(1));
-    expect(foo.detachedCallbackCount, equals(1));
-    expect(foo.attributeChangedCallbackCount, equals(1));
-  });
-}
-</script>
-</sky>