Revert recent changes with helm-set-local-variable (#568).
* helm-elisp.el: Don't use helm-set-local-variable for now.
* helm-files.el: Don't use helm-set-local-variable for now.
* helm-grep.el: Don't use helm-set-local-variable for now.
* helm-mode.el: Don't use helm-set-local-variable for now.