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

[Aleksey Shipilev]

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

-- 
Thanks,
-Aleksey