RFR: 8198432: Remove Thread extension point

[Stefan Johansson]

Hi,

Please review this small change to remove a now unused extension point.

Links
JBS: https://bugs.openjdk.java.net/browse/JDK-8198432
Webrev: http://cr.openjdk.java.net/~sjohanss/8198432/00/

Summary
The Thread class extension support made it possible to add extra data to 
it. This is no longer needed since the internal code using it has been 
removed.

Testing
Built locally and through Mach5.

Thanks,
Stefan