Stop linking `gsha256sum` and add function to remove existing links
Stop linking `gsha256sum` and add function to remove existing links

This is not used as far as I know and is, therefore, just adding
unnecessary complexity.

The new `_remove_sha256sum_link` function should be removed as soon as
all links are gone from machines using this configuration, and the
function includes a console-printed message stating that.

alphabetum/dotfiles,alphabetum/dotfiles,alphabetum/dotfiles,alphabetum/dotfiles