Drop the native FastNewBlockContextStub.

This code is almost never executed in real world and benchmarks,
and there's obviously absolutely no need to have this native code
hanging around for no benefit.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/148873002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18880 ce2b1a6d-e550-0410-aec6-3dcde31c8c00