Clean up g1ParScanThreadState a bit

Suggestiong the following changes in g1ParScanThreadState.hpp:

- remove declaration of handle_evacuation_failure_notifications() without definition
- move inlined code for enqueue_card_if_tracked() in .hpp file into .inline.hpp file
- factor out write barrier during gc code from G1ParScanThreadState::do_oop_evac() for future re-use in JDK-8271880

Found during working on JDK-8271880.