Remove rainbow-mode local variables

This is user-level config, so you should just put something like this in your init code instead:

```el
  (defun sanityinc/enable-rainbow-mode-if-theme ()
    (when (string-match "\\(color-theme-\\|-theme\\.el\\)" (buffer-name))
      (rainbow-mode 1)))

  (add-hook 'emacs-lisp-mode-hook 'sanityinc/enable-rainbow-mode-if-theme)
```