RFR (S) 8233387: Shenandoah: passive mode should disable pacing ergonomically

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8233387

This is the follow-up from JDK-8232791. There, we have added the ergonomic disable block in
ShenandoahPassiveHeuristics. But, there is already the disable block in ShenandoahPassiveMode! In
fact, it was there in ShenandoaPassiveHeuristics before introducing ShenandoahPassiveMode. We should
revert JDK-8232791, and set the flag ergonomically in ShenandoahPassiveMode.

Fix:
  https://cr.openjdk.java.net/~shade/8233387/webrev.01/

Testing: eyeballing GC logs, hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey