RFR: 8058265: No callers of ReferenceProcessor::clear_discovered_references

[Kim Barrett]

Please review this removal of some unused or unneeded code in the
ReferenceProcessor class.

Removed unused ReferenceProcessor::clear_discovered_references() - the
no-arg overload; the 1-arg overload is retained.

Removed vacuous non-product-only ReferenceProcessor::verify_ok_to_handle_reflists. 

Removed ReferenceProcessor::abandon_partial_discovered_list, changing
the only call to instead use equivalent 1-arg clear_discovered_references.

CR:
https://bugs.openjdk.java.net/browse/JDK-8058265

Webrev:
http://cr.openjdk.java.net/~kbarrett/8058265/webrev.00/

Testing:
JPRT