RFR: 8228364: Shenandoah: Remove unused code from ShenandoahBarrierSetC1

[Roman Kennke]

In ShBSC1, needs_null_check is not needed because it is always true, and
the CodeEmitInfo is never used for anything. Both look like left-overs
from WB.

This prepares for a larger refactoring of ShBSC1 and related code.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8228364
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8228364/webrev.00/

Testing: hotspot_gc_shenandoah (x86/aarch64)

Ok?

Roman