RFR(s): 8066822: Remove PSMarkSweep::set_reference_processor

[Kim Barrett]

Note: I will need a sponsor for this change.

Please review this small change to remove an unused function.  This
function has had a "delete this function" comment since the dawn of
the mercurial age.

While I was in the vicinity, I made a couple of additional code
cleanups:

* removed a couple of unnecessary
  debug_only(public:)
    ...
  debug_only(private:)
member exposure blocks that were cluttering up the code.

* Removed some unnecessary upcasts of return values to match the
return type of the function being returned from.

CR:
https://bugs.openjdk.java.net/browse/JDK-8066822

Webrev:
http://cr.openjdk.java.net/~kbarrett/8066822

Testing: JPRT