RFR (sh/jdk11): Follow-up fixes and cleanups for Shenandoah SATB

[Roman Kennke]

This fixes two issues:
- Rename SATBMarkQueue -> ShenandoahSATBMarkQueue in aarch64 code
- Remove obsolete G1CollectedHeap::requires_marking()

http://cr.openjdk.java.net/~rkennke/shjdk11-satb-followup/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?

Roman