RFR: 8242082: Shenandoah: Purge Traversal mode

[Roman Kennke]

Traversal mode has become cumbersome to maintain. Every single
non-trivial feature required extra work to play with Traversal, and
usually caused a trail of follow-up issues. And all that for little
gain. We are transplanting the interesting properties to the upcoming
I-U mode (which is much less trouble-some), and can ditch Traversal mode
altogether.

My original plan was to ditch it after I-U is in, but now I want to do a
couple of cleanups before I-U and I'm not in the mood to special-case
Traversal there. Let's get rid of it ahead of time.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8242082
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8242082/webrev.00/

Testing: hotspot_gc_shenandoah builds and tests fine.

Ok?

Roman