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

[Aleksey Shipilev]

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

-- 
Thanks,
-Aleksey