[15] RFR 8245827: Shenandoah: Cleanup Shenandoah code root iterators and root scanner

[Zhengyu Gu]

ShenandoahCsetCodeRootsIterator and ShenandoahCSetRootScanner are only 
relevant to Traversal GC, which was removed by JDK-8242082, they should 
be removed as well.

After removal, we can rename ShenandoahAllCodeRootsIterator and 
ShenandoahAllRootScanner back to ShenandoahCodeRootsIterator and 
ShenandoahRootScanner, respectively.


Bug: https://bugs.openjdk.java.net/browse/JDK-8245827
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8245827/webrev.00/index.html

Test:
   hotspot_gc_shenandoah


Thanks,

-Zhengyu