RFR: Minor cleanup of Shenandoah SA

[Zhengyu Gu]

- Delete an obsoleted/commented out line.
- Rename SHENANDOAH_HEAP to SHENANDOAH to be consistent with other GCs.

Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/minor_sa_fix/webrev.00/

Test:

   hotspot_serviceability, gc_shenandoah_tier1

Thanks,

-Zhengyu