RFR: JDK-8227199: Shenandoah: Remove superfluous calls to can_do_traversal_gc()

[Roman Kennke]

 We've got a few calls to can_do_traversal_gc() in assembler which are
superfluous because ShenandoahSATBBarrier is already checked and covers
the traversal case. Let's remove them.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8227199
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8227199/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?

Roman