RFR(T) 8225514: Shenandoah: ShenandoahCodeRoots should inherit from AllStatic

[Zhengyu Gu]

A trivial cleanup.

Bug: https://bugs.openjdk.java.net/browse/JDK-8225514
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8225514/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu