object_is_smi was always false so there's no need to actually have it.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/19541007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15774 ce2b1a6d-e550-0410-aec6-3dcde31c8c00