(XS) RFR: 8134642: ParkEvent::RawThreadIdentity appears to be unused and should be removed.

[David Holmes]

Bug:

https://bugs.openjdk.java.net/browse/JDK-8134642

Webrev:

http://cr.openjdk.java.net/~dholmes/8134642/webrev/

Simple removal of unused variables.

Testing: JPRT

Will run performance check to ensure field padding changes don't cause 
performance regression.

Thanks,
David