RFR: 8144992: Remove OopIterateClosure::idempotent

[Stefan Karlsson]

Hi all,

Please review this patch to remove the OopIterateClosure::idempotent() 
function.

http://cr.openjdk.java.net/~stefank/8144992/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8144992

There's no closure that overrides idempotent() anymore, so this patch 
removes the function an all usages.

Thanks,
StefanK