RFR: 8198430 : Remove G1Allocator extension point

[Stefan Johansson]

Hi,

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

Links
JBS: https://bugs.openjdk.java.net/browse/JDK-8198430
Webrev: http://cr.openjdk.java.net/~sjohanss/8198430/00/

Summary
The G1Allocator extension support made it possible to have different 
allocators used by G1. This is no longer needed since the internal code 
using it has been removed.

Testing
Built locally and through Mach5.

Thanks,
Stefan