unp: remove unfree unrar from the default backend list
unp: remove unfree unrar from the default backend list

`unrar` is unfree, meaning `unp` cannot be built by default if `unrar`
is in its dependencies.

A simple

  env NIXPKGS_ALLOW_UNFREE=1 nix-shell -p unrar

will make `unp` work with .rar files.

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