[15] RFR (T) 8240915: Shenandoah: Remove unused fields in init mark tasks

[Zhengyu Gu]

Please review this trivial change that removes unused fields in 
ShenandoahInitTraversalCollectionTask and ShenandoahInitMarkRootsTask.

Bug: https://bugs.openjdk.java.net/browse/JDK-8240915
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8240915/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu