Get rid of the unused native code StringAddStub.

BUG=v8:2990
LOG=n
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/144023009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18752 ce2b1a6d-e550-0410-aec6-3dcde31c8c00