RFR: 8075401: Remove DiscoveredListIterator::update_discovered()

[Kim Barrett]

Please review this change to remove unnecessary calls to
DiscoveredListIterator::update_discovered(), and remove the now unused
function. Details are in the CR.

I will need a sponsor for this change.

CR:
https://bugs.openjdk.java.net/browse/JDK-8075401

Webrev:
http://cr.openjdk.java.net/~kbarrett/8075401/webrev.00/

Testing:
JPRT, Aurora ad hoc GC Nightly and other tests,
local JTREG of hotspot/tests/[closed/]{runtime,gc},
RefWorkload with -XX:+UseG1GC, -XX:+UseParallelOldGC, -XX:+UseConcMarkSweepGC