RFR: Verifier should not assert cset in forwarded test block

[Aleksey Shipilev]

Small fix:
 http://cr.openjdk.java.net/~shade/shenandoah/verifier-fwd-no-cset/webrev.01/

We don't need to do this on _verify_forwarded path, because it is handled below
on _verify_cset path.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey