RFR: 8198957: Remove unused function ArgumentsExt::set_gc_specific_flags

[Erik Helin]

Hi all,

this small patch removes the unused function 
ArgumentsExt::set_gc_specific_flags.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8198957

Patch:
http://cr.openjdk.java.net/~ehelin/8198957/00/

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

Thanks,
Erik