RFR [9]: Purge partial GC and matrix code

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/purge-partial-9/webrev.01/

sh/jdk8u does not have partial and matrix code. sh/jdk9 has STW partial and matrix code. sh/jdk10
has concurrent partial and matrix code. The backports from sh/jdk10 to sh/jdk9 are now very
complicated, because they require touching shared code in 9 to fit the changes in barriers (e.g.
keep_alive_barrier).

Knowing that sh/jdk9 is our backport repository, and the roadmap for JDK release cadence that
probably skips lots of sh/jdk9 deployments, and that partial is the experimental feature, I propose
we purge the partial and matrix code from sh/jdk9. This cuts down the backporting and support work
significantly, and keeps our intrusive experimental features contained in sh/jdk10.

Thoughts?

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey