Remove shell.nix

This change removes support for entering development shell. It isn't
that useful in this case and it still can be done if you try hard enough
(and if you read the header of nix-book.sh).

Author: meator

nix-book.sh

@@ -1,4 +1,6 @@
 #!/usr/bin/env nix-shell
 #! nix-shell -i bash --pure
+#! nix-shell -p bash mdbook mdbook-toc mdbook-admonish
+#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/tarball/nixos-24.05
 
 exec ./book.sh "$@"