RFR (S) 8237570: Shenandoah: cleanup uses of allocation/free threshold in static heuristics

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8237570

Fix:
  https://cr.openjdk.java.net/~shade/8237570/webrev.01/

As noted by Justinas in the separate thread, ShAllocThresh has no effect on "static" heuristics, so
it should not be adjusted. Also, it should use ShMinFreeThresh, as other heuristics use. This makes
ShFreeThresh unused, and it is removed for clarity.

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey