[15] RFR(T) 8246342: Shenandoah: remove unused ShenandoahIsMarkedNextClosure

[Zhengyu Gu]

Please review this trivial patch that removes unused 
ShenandoahIsMarkedNextClosure.

Bug: https://bugs.openjdk.java.net/browse/JDK-8246342
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8246342/webrev.00/

Test:
   Built on Linux x86_64

Thanks,

-Zhengyu