RFR: Remove obsolete and unused reduce-storeval-barrier optimization code

[Roman Kennke]

This is an almost 1:1 revert of an earlier changeset that attempted to 
optimize storeval barriers. The idea back then was that when we check 
for conc-mark-in-progress anyway, we could just as well only invoke the 
storeval (i.e. read) - barrier if conc-mark is active. It never worked 
very well (and we never turned it on by default) and gained us anything 
significant. But now it's even worse: updating-refs now also happens 
outside conc-mark, and thus the whole idea is obsolete. In fact, maybe 
we can do something smarter once all the X_in_progress flags have been 
folded into the gc-phase-field (need to see this when it's there). The 
patch removes the code, and thus reduces our upstream diff.

Test: hotspot_gc_shenandoah

http://cr.openjdk.java.net/~rkennke/remove-reduce-storeval/webrev.00/ 
<http://cr.openjdk.java.net/%7Erkennke/remove-reduce-storeval/webrev.00/>

Ok?