RFR (S) 8240070: Shenandoah: remove obsolete ShenandoahCommonGCStateLoads

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8240070

This is the leftover of the older experiment that optimized the frequently emitted barriers. With
the switch to LRB and questionable performance improvements (sometimes hijacked by elevated register
pressure), it makes less sense to keep the option exposed and C2 code more complicated.

Removal webrev:
  https://cr.openjdk.java.net/~shade/8240070/webrev.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey