RFR (S) 8240868: Shenandoah: remove CM-with-UR piggybacking cycles

[Aleksey Shipilev]

RFR:
  https://bugs.openjdk.java.net/browse/JDK-8240868

See the rationale in description.

Webrev:
  https://cr.openjdk.java.net/~shade/8240868/webrev.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey