RFR(T) 8222403: Shenandoah: Remove ShenandoahAlwaysTrueClosure, use AlwaysTrueClosure instead

[Zhengyu Gu]

Please review this trivial cleanup, to remove deplicated closure.

Bug: https://bugs.openjdk.java.net/browse/JDK-8222403
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8222403/webrev.00/

Test:
   hotspot_gc_shenandoah: Linux 64 (fastdebug and release)

Thanks,

-Zhengyu