RFR: 8198433: Remove WhiteBox 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-8198433
Webrev: http://cr.openjdk.java.net/~sjohanss/8198433/00/

Summary
The WhiteBox testing API extension support made it possible to add 
specialized native code for some test methods. This is no longer needed 
since the internal code using it has been removed.

Testing
Built locally and through Mach5.

Thanks,
Stefan