The indirection through our nixpkgs fork for PaymentServer packaging only creates extra work

PaymentServer is packaged for Nix using haskell.nix which effectively pins its own nixpkgs. Placing PaymentServer in-tree with our nixpkgs fork doesn't actually cause it to use much (or anything?) from that nixpkgs tree.

Instead, it gives us an extra place to update whenever we want to update PaymentServer. We have to update it in our nixpkgs fork and then we have to update our pin of our nixpkgs fork.

We should just refer directly to PaymentServer from PrivateStorageio, instead.