RFR: Remove ShenandoahWriteBarrierCsetTestInIR flag

[Roman Kennke]

This is actually a prerequisite to removing the wb-stub for C2, because
if we don't call via the stub, we can't do the cset there. We've no use
for this flag anyway, doing cset test in ideal is what we do since a
long time. Let's remove it.

http://cr.openjdk.java.net/~rkennke/remove-cset-in-ideal-flag/webrev.00/

Testing: tier3_gc_shenandoah

OK?

Roman