RFR: Make concurrent precleaning log message optional again

[Roman Kennke]

Am 22.01.2018 um 10:16 schrieb Aleksey Shipilev:
> http://cr.openjdk.java.net/~shade/shenandoah/preclean-optional/webrev.01/
> 
> This is the fix for UX regression after recent refactoring: even when precleaning is not enabled
> and/or process references is not enabled, we still print "Concurrent precleaning" message in the log.
> 
> Testing: hotspot_gc_shenandoah
> 
> Thanks,
> -Aleksey
> 

Ok