RFR (S) 8240217: Shenandoah: remove ShenandoahEvacAssist

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8240217

ShenandoahEvacAssist is an experimental option that strived to make calling into WB/LRB slowpath
less frequent.

It implicitly relied on WB/LRB midpath to check for forwardee and shortcut from there. With the
introduction of self-fixing barriers, this was intentionally removed. Therefore, Shenandoah would
call into slow-path anyway, even when evac-assist path had evacuated some objects.

Also, with Traversal, the assist path breaks out of Traversal's intent to evacuate the objects in
traversal order. There, it becomes actively harmful. We should consider removing it.

Webrev:
  https://cr.openjdk.java.net/~shade/8240217/webrev.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey