remove unnecessary 'src = null' in mkDerivation

artiq/artiq-board.nix

@@ -47,7 +47,6 @@ let
 in pkgs.python3Packages.buildPythonPackage rec {
   name = "artiq-board-${target}-${variant}-${version}";
   version = import ./pkgs/artiq-version.nix;
-  src = null;
   phases = [ "buildPhase" "installPhase" ];
   buildPhase = 
     ''

artiq/windows/manual-test-run.nix

@@ -17,7 +17,6 @@ in
 
 stdenv.mkDerivation {
   name = "windows-test";
-  src = null;
 
   phases = [ "installPhase" "checkPhase" ];
   installPhase = "touch $out";

main.nix

@@ -55,7 +55,6 @@ in
 
     windows-no-hardware-tests = pkgs.stdenv.mkDerivation {
       name = "windows-no-hardware-tests";
-      src = null;
       buildInputs = [ (windowsRunner {}) ];
       phases = [ "buildPhase" ];
       buildPhase = ''