RFR(S): Clean-up BarrierSetC2

[Nils Eliasson]

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