RFR: 6979279: remove special-case code for ParallelGCThreads==0

[Marcus Larsson]

Hi,

Can I please have reviews for the following patch removing the 
special-case code used when ParallelGCThreads is 0. A recent change [1] 
forbidding this flag to be set to 0 turns this special handling into 
dead code, which should be removed.

Functions with two interfaces for single threaded or parallel execution 
used exclusively in these cases have been merged to a single parallel 
version (for example G1RemSet's scrub and scrub_par).

Webrev:
http://cr.openjdk.java.net/~mlarsson/6979279/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-6979279

Testing:
- jprt
- Aurora
   - Weblogic
   - runThese
   - Kitchensink
   - jtreg
   - VM GC nightly

Thanks,
Marcus

[1]: https://bugs.openjdk.java.net/browse/JDK-8059527