RFR: 8031401: Remove unused code in the reference processor

[Kim Barrett]

Please review this change to remove JVM code to support compatibility
with old (pre-JDK-4243978 fix) versions of the JDK.

All of the JVM code that was conditional on various forms of
pending_list_uses_discovered_field now assume that to be true, with
the code for the false case being deleted.

JDK_Version::initialize() now examines the
pending_list_uses_discovered_field configuration bit provided by the
JDK, and calls vm_exit_during_initialization if the bit is not set,
e.g. the JDK is not claiming to provide that behavior.
[Given that we now build JDK and JVM together, it's not clear that we
still need this and other similar configuration tests, but I'm going
to leave that to someone else to worry about.]

As part of this change, removed DiscoveredListIterator::make_active
and the call to it from ReferenceProcessor::process_phase1.
make_active is problematic because:

- Only active references are ever discovered. So when the reference
was added to the list it had a NULL next field.

- The only place that deactivates a reference is enqueue (either by
the collector or by Java code).

- For a STW collector, there can't be an enqueue operation (of either
kind) between discovery and process_phase1, so the next field is
necessarily still NULL.

- For a concurrent collector, gc enqueuing doesn't occur between
discovery and process_phase1, but Java enqueuing could. But if the
reference has been enqueued, the next field is the link in the queue
list. Bashing it to NULL here is very bad.

make_active() seems to be a lingering artifact of
!pending_list_uses_discovered_field, and should have been
conditionalized on that by the fix for JDK-4965777.  It would have
been OK and even correct for a STW collector, but it has always been
wrong for a concurrent collector.

CR:
https://bugs.openjdk.java.net/browse/JDK-8031401

Webrev:
http://cr.openjdk.java.net/~kbarrett/8031401/webrev.00/

Testing:
JPRT, RefWorkload with G1, CMS, and ParOld.
Aurora ad hoc GC Nightly & Quick tests