RFR: 8200169: Flatten G1Allocator class hierarchy

[Stefan Johansson]

Hi,

Please review this change to remove some unnecessary class hierarchies 
in G1.

Links
JBS: https://bugs.openjdk.java.net/browse/JDK-8200169
Webrev: http://cr.openjdk.java.net/~sjohanss/8200169/00/

Summary
After the removal of several extension points and some other cleanups in 
G1, there is no longer any need for the two class hierarchies in 
g1Allocator.hpp. These can be flattened to only consist of two concrete 
classes instead.

Testing
* Built both locally and through mach5.
* hs-tier1 through mach5

Thanks,
Stefan