RFR: Wipe out ShenandoahStoreCheck implementation

[Aleksey Shipilev]

Our experimental ShenandoahStoreCheck implementation has been with us for about two years, and we
have never seen it fail. It makes little benefit to keep it on many paths in shared code, especially
in .ad files. It is not complete as well: there are paths not covered by this thing, and completing
it would balloon up our upstream exposure.

We also have the idea to reimplement this with optional aliased heap [1], which would hopefully be
less intrusive. For that, we keep the flag around.

This patch wipes out ShenandoahStoreCheck implementation:
  http://cr.openjdk.java.net/~shade/shenandoah/store-check-wipe/webrev.01/

Testing: x86_64 and aarch64 builds, x86_64 hotspot_fast_gc_shenandoah

Thanks,
-Aleksey

[1] http://mail.openjdk.java.net/pipermail/shenandoah-dev/2018-January/004816.html