nix: remove the development flake

I don't use NixOS anymore and thus cannot keep up the flake provided for
development.

flake.lock

@@ -1,26 +0,0 @@
-{
-  "nodes": {
-    "nixpkgs": {
-      "locked": {
-        "lastModified": 1636267212,
-        "narHash": "sha256-KDS173KqmqrYUPY9N4vf750GxIo+S6E0djyq2BsQm8s=",
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "c935f5e0add2cf0ae650d072c8357533e21b0c35",
-        "type": "github"
-      },
-      "original": {
-        "id": "nixpkgs",
-        "ref": "nixos-unstable",
-        "type": "indirect"
-      }
-    },
-    "root": {
-      "inputs": {
-        "nixpkgs": "nixpkgs"
-      }
-    }
-  },
-  "root": "root",
-  "version": 7
-}

flake.nix

@@ -1,21 +0,0 @@
-{
-  description = "Nix flake for dumb-jose development.";
-
-  inputs = {
-    nixpkgs.url = "nixpkgs/nixos-unstable";
-  };
-
-  outputs = { self, nixpkgs }:
-    let
-      pkgs = nixpkgs.legacyPackages.x86_64-linux;
-    in
-    {
-      devShell.x86_64-linux = pkgs.mkShell {
-        buildInputs = with pkgs; [
-          delve
-          go_1_17
-          gopls
-        ];
-      };
-    };
-}