Remove nix shell

[iosmanthus]

Feature Request

Remove the nix-shell file since the dev shell is really simple, just depends on maven and jdk8. If you want to create a dev shell, use the following snippet:

{
  description = "Java Client for TiKV";
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/master";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem
      (
        system:
        let
          repo = "client-java";
          pkgs = import nixpkgs {
            inherit system;
          };
        in
        {
          devShell = pkgs.mkShell {
            hardeningDisable = [ "all" ];
            buildInputs = with pkgs;[ jdk8 maven ];
        }
      );
}