RFR: 8197843: Remove unused method G1EvacuationRootClosures::create_root_closures_ext

[Erik Helin]

Hi all,

this patch removes the unused method 
G1EvacuationRootClosures::create_root_closures_ext. Since this method is 
the sole content of the file g1RootClosures_ext.cpp, that file is also 
removed.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8197843

Patch:
http://cr.openjdk.java.net/~ehelin/8197843/00/

Testing:
- make run-test-tier1 on Linux x86-64

Thanks,
Erik