Revert "ES6: Implement Object.setPrototypeOf"

This reverts commit r18685 for breaking WebKit tests.

TBR=arv@chromium.org

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18687 ce2b1a6d-e550-0410-aec6-3dcde31c8c00