Simplify attribute iteration, remove unreachable code

For the unreachable code:
- This code was unreachable because it required `keys` to be empty,
  in which case the `cnt_omitted_private` and `cnt_omitted_methods`
  values would never have a chance to be incremented.
- This appears to have been intended to be a UX nicety to tell the user
  why nothing showed up under an item when they expanded iteration
- The markers (e.g. "[pri+()]") that are added on expansion already
  provide a useful hint for these purposes, and are also already
  consistent when some, but not all, attributes are shown.