helmfile: Stop prefixing PATH with kubernetes-helm

So helmfile will work with helm plugins wrapped by pkgs.wrapHelm:

```
{
inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
inputs.flake-utils.url = "github:numtide/flake-utils";

outputs = { self, nixpkgs, flake-utils }:
flake-utils.lib.eachDefaultSystem (system: let
pkgs = nixpkgs.legacyPackages.${system};
helm = pkgs.wrapHelm pkgs.kubernetes-helm {
plugins = with pkgs.kubernetes-helmPlugins; [
helm-diff helm-secrets
];
};
in {
devShell = pkgs.mkShell {
nativeBuildInputs = [
helm
pkgs.helmfile
];
};
});
}
```

pkgs/applications/networking/cluster/helmfile/default.nix

···
-{ lib, buildGoModule, fetchFromGitHub, makeWrapper, kubernetes-helm }:
+{ lib, buildGoModule, fetchFromGitHub }:
 
 buildGoModule rec {
   pname = "helmfile";
···
 
   doCheck = false;
 
-  nativeBuildInputs = [ makeWrapper ];
-
   subPackages = [ "." ];
 
   ldflags = [ "-s" "-w" "-X github.com/roboll/helmfile/pkg/app/version.Version=${version}" ];
-
-  postInstall = ''
-    wrapProgram $out/bin/helmfile \
-      --prefix PATH : ${lib.makeBinPath [ kubernetes-helm ]}
-  '';
 
   meta = {
     description = "Deploy Kubernetes Helm charts";