Revert "fix mkpasswd: use the git repository"

This reverts commit ce13d178562c585bf1e10eb15350465460976294 since it
breaks evaluation:

in job ‘nixpkgs’:
anonymous function at /nix/store/f73d4b03nsmccb1w5b1qfgc7dpvx35gw-git-export/pkgs/tools/security/mkpasswd/default.nix:1:1 called without required argument `fetchFromGitHub', at /nix/store/f73d4b03nsmccb1w5b1qfgc7dpvx35gw-git-export/lib/customisation.nix:58:12