ES6: Remove __proto__ setter poison pill

http://people.mozilla.org/~jorendorff/es6-draft.html#sec-set-object.prototype.__proto__

The __proto__ setter should be reusable on other objects.

BUG=v8:2804
LOG=y
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/103343005

Patch from Erik Arvidsson <arv@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19165 ce2b1a6d-e550-0410-aec6-3dcde31c8c00