RFR: 8198431: Remove G1FullCollector 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-8198431
Webrev: http://cr.openjdk.java.net/~sjohanss/8198431/00/

Summary
The G1 Full GC extension support made it possible to have a different 
policy for creating compaction queues. This is no longer needed since 
the internal code using it has been removed.

Testing
Built locally and through Mach5.

Thanks,
Stefan