RFR: 8249884: Shenandoah: Call report_num_dead() from ShParallelWeakRootsCleaningTask destructor

[Roman Kennke]

JDK-8249877 added a call to report_num_dead() after each call to
ShParallelWeakRotsCleaningTask usage. It is cleaner to simply call it
from the destructor of that task. 

Bug:
https://bugs.openjdk.java.net/browse/JDK-8249884
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8249884/webrev.00/

Testing: hotspot_gc_shenandoah, MemberNameLeak.java

Ok?

Thanks,
Roman