PackWriter/Statistics: Remove the bitmapt hit stats

PackWriter/Statistics: Remove the bitmapt hit stats

The request uses bitmaps for reachability and to decide what to
pack. Setting the listener in the PackWriter only covers the second
case.

Remove the listener from the PackWriter. It makes more sense to set it
in the reader and at the moment the BitmapIndex only supports a single
listener.

This was introduced after the 6.8 tag, so it should be safe to remove.

Change-Id: Ib4c0a805c70e54b6a9e7f78ccdb8e0a465d238f1