RFR(S): Clean-up BarrierSetC2

[Roman Kennke]

Looks good to me, too (lots of this stuff was for Shenandoah pre-LRB).

Thanks,
Roman

> Hi,
> 
> I can across a lot of dead code in BarrierSetC2 while debugging a
> completely separate problem. This is leftovers from legacy barrier
> implementations that no longer is needed.
> 
> This is a quick cleanup where non-used parts are removed. I tried to
> keep it as simple as possible to avoid introducing any bugs. There are
> additional cleanups that can be done in these files later.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8235653
> 
> Webrev: http://cr.openjdk.java.net/~neliasso/8235653/webrev.01
> 
> 
> Please review,
> 
> Nils Eliasson
>