Review request: 8012687: Remove unused is_root checks and closures

[Stefan Karlsson]

http://cr.openjdk.java.net/~stefank/8012687/webrev.00/

 From bug description:
There's an unused parameter named is_root that is passed down to a 
number of MarkSweep/ParOld closures. This enhancement suggests that we 
remove it and cleanup the trails after it.

thanks,
StefanK