[8] RFR: Fix Zero build after LRB backport moves, remove other stubs

[Roman Kennke]

Ok.

Thanks,
Roman

> https://cr.openjdk.java.net/~shade/shenandoah/8u-lrb-fix-zero/webrev.01/
> 
> 8u is very sensitive to build configuration for platforms that do not enable Shenandoah. This patch
> rewires the build to succeed when C1/C2 is disabled, like in Zero. Also removes the obsolete
> shenandoahBarrierSet implementations that were there for alternative platforms.
> 
> Testing: x86_64 {server, zero} builds; x86_64 server hotspot_gc_shenandoah
>