Drop obsolete ES6 TODOs about activating extended mode.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/338363002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21877 ce2b1a6d-e550-0410-aec6-3dcde31c8c00