RFR (S) 8241692: Shenandoah: remove ShenandoahHeapRegion::_reserved

[Roman Kennke]

Yes, looks good!

Roman


> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8241692
> 
> Follow up from JDK-8241668: _reserved field is not actually needed, because we can just use bottom()
> and end() available. Saves 16 bytes per region.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8241692/webrev.01/
> 
> Testing: hotspot_gc_shenandoah
>