[15] RFR 8238162: Shenandoah: Remove ShenandoahTaskTerminator wrapper

[Zhengyu Gu]

I can not recall why we still have terminator wrapper, probably a 
leftover after we upstreamed OWST terminator. Let's remove it.

Bug: https://bugs.openjdk.java.net/browse/JDK-8238162
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8238162/webrev.00/index.html

Test:
   hotspot_gc_shenandoah


Thanks,

-Zhengyu