go: stop setting GOPATH

The compiler does not need it anymore, has not needed it for many years
iirc. This just goes in and pollutes the environment overriding the
users GOPATH and causing grief.

Go even warns about it itself, without vs with this commit:

```sh
~> go env GOPATH
/home/manny/go
~> nix-shell -p go
~> go env GOPATH
warning: GOPATH set to GOROOT (/nix/store/gvw1mfpdrk7i82884yhxf9lf5j3c12zm-go-1.14.1/share/go) has no effect
/nix/store/gvw1mfpdrk7i82884yhxf9lf5j3c12zm-go-1.14.1/share/go
~> exit
~> nix-shell -I nixpkgs=cloned/NixOS/nixpkgs -p go
~> go env GOPATH
/home/manny/go
~> exit
```

pkgs/development/compilers/go/1.14.nix

@@ -229,8 +229,6 @@ stdenv.mkDerivation rec {
     runHook postInstall
   '';
 
-  setupHook = ./setup-hook.sh;
-
   disallowedReferences = [ goBootstrap ];
 
   meta = with stdenv.lib; {

pkgs/development/compilers/go/1.4.nix

@@ -151,8 +151,6 @@ stdenv.mkDerivation rec {
     ./all.bash
   '';
 
-  setupHook = ./setup-hook.sh;
-
   meta = with stdenv.lib; {
     branch = "1.4";
     homepage = "http://golang.org/";