RFR: Make concurrent precleaning log message optional again

[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