RFR (S) 8240070: Shenandoah: remove obsolete ShenandoahCommonGCStateLoads

[Roman Kennke]

As far as I understand, this optimization pass would not work with LRB
anyway. So yeah, please remove it.

Roman

> 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}
>