Update geiser-unload to clean up scheme-mode-hook and auto-mode-alist

I've updated the `geiser-unload` defun in [geiser-reload.el](https://github.com/jaor/geiser/blob/master/elisp/geiser-reload.el) to remove the additions to `scheme-mode-hook` and `auto-mode-alist` from [geiser.el](https://github.com/jaor/geiser/blob/master/elisp/geiser.el).  

I've found these changes helpful for using Scheme Mode after unloading Geiser.