Remove the special case handling for the topmost/bottommost
rows in each processing unit. This causes slightly different
effects depending on whether striped-loop-restoration is enabled.

With striped-loop-restoration:
  Now that we explicitly fill out 3 rows of above/below pixels
  for each stripe, we don't need to use stepdown_wiener_kernel.
  Instead, the duplication of the topmost/bottommost pixels
  accomplishes the same task, while making the code much cleaner.

  This patch should not cause a change in output, except in a
  couple of cases which were already questionable. In particular,
  it fixes bug #953, where the Wiener filter could not handle
  small processing units (<4 rows high)

Without striped-loop-restoration:
  The Wiener filter returns to using a full 3 pixels above/below
  the processing unit. In order to make sure there are enough
  pixels, we need to expand WIENER_BORDER_VERT to 3 pixels.

  This will result in a slight change in output, but should be
  fairly minor.

BUG=aomedia:953

Change-Id: I9530ef55909246f7ba488b7ecfd92d59e776b2f9