Remove RegExp.$input.

R=arv@chromium.org, mathiasb@opera.com
BUG=v8:3486
LOG=Y

Review URL: https://codereview.chromium.org/465273002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23223 ce2b1a6d-e550-0410-aec6-3dcde31c8c00