RFR: Remove obsolete code snippet related to barriers on constants

[Roman Kennke]

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/

The patch for 8u is identical, except for line-numbers-offset and paths.

Testing: hotspot_gc_shenandoah (8 and 11)

Ok?

Roman