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

[Roman Kennke]

Makes sense. Good!

Thanks,
Roman

> 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
>