Minor clean-ups of the reference processing code.

Minor clean-ups of the reference processing code.

* Eliminate two dead stores to a pendingNextOffset variable.  This
  value is only needed in the abstractions that delaying, enqueue and
  dequeue of references.

* Clean-up the comment that preceeds the reference list walking.  This
  comment was originally copied from the code that preserves soft
  references.  That routine walks the list, but the clearing code
  actually unlinks the list.

* Remove the the dirtyObjectVisitor.  This does the exact same thing
  as the existing scanObject code.

Change-Id: I20b68e52e4147aa602dd317392f68809ba4d7c63