RFR: Remove obsolete code snippet related to barriers on constants

[Aleksey Shipilev]

On 12/12/18 6:19 PM, Roman Kennke wrote:
> In JDK11 and JDK8u, subnode.cpp, there is a Shenandoah-only block of
> code that was there to support barriers on constants. Barriers on
> constants have been removed since a while, let's remove those blocks too.
> 
> http://cr.openjdk.java.net/~rkennke/remove-const-barrier-code/webrev.00/

OK.

-Aleksey