RFR: Fix merged update refs

[Roman Kennke]

I made a little mistake yesterday when I moved flag initialization into
ShenandoahHeuristic: I forgot to remove the _update_refs_adaptive field
in AdaptiveHeuristic, and this one overrides the same field in
ShenandoahHeuristics. Problem is the forgotten field never gets
initialized and is always false.

http://cr.openjdk.java.net/~rkennke/fix-merge-uprefs/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/fix-merge-uprefs/webrev.00/>

Ok?

Roman