RFR(S): 8027746: Remove do_gen_barrier template parameter in G1ParCopyClosure

[Thomas Schatzl]

Hi all,

  could I have a few reviews for the following simple cleanup?

The do_gen_barrier template parameter of G1ParCopyClosure is not used,
so it has been removed. Further it removes one level of the
G1ParCopyClosure from the G1 oop closure hierachy that has been
obsoleted by the NPG changes.

In total it's about removing 133 lines of code, and changing a few to
accommodate the removals.

This patch is based on the changes for JDK-8027454 just posted.

Webrev:
http://cr.openjdk.java.net/~tschatzl/8027746/

CR:
https://bugs.openjdk.java.net/browse/JDK-8027746

Testing:
jprt, FMW apps, specjbb2005, specjbb2013, dacapo, and lots
of other testing in the recent months

Thanks,
  Thomas