RFR (11): Clean up unused shenandoah_wb_C leftovers

[Roman Kennke]

This is a little cleanup in sh/jdk11. We have leftover use of
shenandoah_wb_C stub. It is not used anymore. This change removes the
relevant code and brings the affected code to state pre-LRB of jdk/jdk.
This is preparatory work for LRB backport to jdk 11.

http://cr.openjdk.java.net/~rkennke/jdk11-cleanup-wb-C/webrev.00/

Testing: hotspot_gc_shenandoah (x86/aarch64)

Good?

Roman