RFR (S): 8224160: Move G1RemSetScanClosure into g1RemSet.cpp file

[Thomas Schatzl]

Hi all,

  can I have reviews for this cleanup that moves the
G1RemSetScanClosure class from a hpp file close to the only use in the
cpp file. This has been made possible by the earlier JDK-8218668
change.

CR:
https://bugs.openjdk.java.net/browse/JDK-8224160
Webrev:
http://cr.openjdk.java.net/~tschatzl/8224160/webrev/index.html
Testing:
hs-tier1

Thanks,
  Thomas