Remove obsolete unchecked accessors in JSFunction.

R=hpayer@chromium.org
BUG=v8:1490

Review URL: https://codereview.chromium.org/17833002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15338 ce2b1a6d-e550-0410-aec6-3dcde31c8c00