npm2nix: remove

`npm2nix` is unmaintained for about two years[1], node2nix[2] or
probably even yarn2nix[3] should be used nowadays.

[1] https://github.com/nixos/npm2nix#disclaimer-this-project-is-no-longer-maintained-instead-use-node2nix
[2] https://github.com/svanderburg/node2nix
[3] https://github.com/moretea/yarn2nix



Signed-off-by: Maximilian Bosch <maximilian@mbosch.me>