RFR (trivial) 8068593: Remove or repurpose unused PerfCounters from objectMonitor

[coleen.phillimore at oracle.com]

Just removed them and ran tier1 tests.   The ones remaining are deemed 
useful.  See bug for details.

open webrev at http://cr.openjdk.java.net/~coleenp/8068593.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8068593

Thanks,
Coleen