function testDelete_RootSimple() {
  var tree = insertToTree(4);
  tree.remove(9);
  tree.remove(17);
  tree.remove(21);
  assertEquals('0[13]\n_{13}_\n', tree.toString());
  tree.remove(22);
  assertEquals('0[13]\n_{13}_\n', tree.toString());
  tree.remove(13);
  assertEquals('0[]\n_{}_\n', tree.toString());
}
Deletes the last few keys from root.

9|13|17|21

Delete 9, 17, 21, and 13. Also tests deleting an non-existent value shall
yield no-op.