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

sky/tests/editing/delete_block_contents.sky

-<sky>
-This test verifies that the height of an editable block remains the same after adding block elements and removing them.
-<div contenteditable="true" style="border: solid blue" id="test"></div>
-<script>
-import "../resources/third_party/unittest/unittest.dart";
-import "../resources/unit.dart";
-
-import "dart:async";
-import "dart:sky";
-
-void main() {
-  initUnit();
-
-  test("remains the same after adding block elements and removing them", () {
-    var elem = document.getElementById("test");
-    var originalHeight = elem.offsetHeight;
-    var d = elem.appendChild(document.createElement('div'));
-    d.appendChild(new Text('aaa'));
-    d = elem.appendChild(document.createElement('div'));
-    d.appendChild(new Text('bbb'));
-    var newHeight = elem.offsetHeight;
-
-    elem.removeChildren();
-
-    new Timer(Duration.ZERO, expectAsync(() {
-      expect(elem.offsetHeight, equals(originalHeight));
-    }));
-  });
-}
-</script>
-</sky>