RFR: 8142399: G1ParCopyClosure does not need do_oop_work

[Erik Helin]

Hi all,

this small patches removes the method do_oop_work from G1ParCopyClosure.
The method is simply not needed because the only thing do_oop_nv did was
to call do_oop_work, so we might as well remove do_oop_work and only
keep do_oop_nv.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8142399

Webrev:
http://cr.openjdk.java.net/~ehelin/8142399/webrev.00/

Testing:
JPRT

Thanks,
Erik