RFR(T) 8225804: SA: Remove unused CollectedHeap.oopOffset() method

[Zhengyu Gu]

Please review this patch that removes a method that is no longer needed.
CollectedHeap.oopOffset() was added for Shenandoah GC to offset extra 
header for forwarding pointer. Recently, Shenandoah removed the header 
and folded the forwarding pointer into regular header, so this method is 
now obsoleted.


Bug: https://bugs.openjdk.java.net/browse/JDK-8225804
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8225804/webrev.00/

Test:
  hotspot_serviceability (fastdebug and release)
  Submit test in progress.

Thanks,

-Zhengyu