RFR: 8134806: Clean up write_ref_field_work

[Kim Barrett]

Please review this small cleanup of BarrierSet::write_ref_field_work.
This change

- Removes an unnecessary pure virtual declaration.

- Removes mostly unused default argument values, adding an explicit
argument to one caller.

- Changes an unnecessarily public declaration to protected.

CR:
https://bugs.openjdk.java.net/browse/JDK-8134806

Webrev:
http://cr.openjdk.java.net/~kbarrett/8134806/webrev.00/

Testing:
jprt