[15] RFR 8234399: Shenandoah: Cleanup native load barrier

[Zhengyu Gu]

Please review this cleanup of a hack, which was added to workaround the 
problem manifested in JDK-8237396.

With JDK-8237396 resolved, let's remove it.

Bug: https://bugs.openjdk.java.net/browse/JDK-8234399
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8234399/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu