Disable regexp highlighting entirely for now.

See https://github.com/defunkt/coffee-mode/commit/be60c13#L0R885

This PR disables regexp highlighting entirely, because reverting brings us back to #110, but I know that this is not ideal. Wish I had stronger Emacs Lisp chops. Perhaps `syntax-propertize-extend-region-functions` / `syntax-propertize-function` could be of help?