RFR: JDK-8141037: Remove _MARKING_VERBOSE_ from the G1 code

[Kim Barrett]

> This is a fix, which is very similar to "JDK-8140781: Remove 
> _MARKING_STATS_ from the G1 code".
>
> http://cr.openjdk.java.net/~brutisso/8141037/webrev.00/
> https://bugs.openjdk.java.net/browse/JDK-8141037

Name and description of G1MarkingVerboseLevel seem wrong now.
Do we even need that (develop) option any more?  After this change it
is only used to control whether ConcurrentMark::print_stats is a nop
or instead calls print_stats on each of its tasks.  Is there perhaps
some other variable more appropriate for controlling task stats printing?

Other than that, looks good.