test/overriding: remove python deprecated stdenv override tests

These tests now throw a warning which is prohibited inside Nixpkgs:
  Passing `stdenv` directly to `buildPythonPackage` or `buildPythonApplication` is deprecated. You should use their `.override` function instead, e.g:
    buildPythonPackage.override { stdenv = customStdenv; } { }

pkgs/test/overriding.nix

@@ -446,20 +446,6 @@ let
         expr = package-stub-libcxx.stdenv;
         expected = pkgs.libcxxStdenv;
       };
-      overridePythonAttrs-stdenv-deprecated = {
-        expr =
-          (package-stub.overridePythonAttrs (_: {
-            stdenv = pkgs.clangStdenv;
-          })).stdenv;
-        expected = pkgs.clangStdenv;
-      };
-      overridePythonAttrs-override-clangStdenv-deprecated-nested = {
-        expr =
-          (package-stub-gcc.overridePythonAttrs {
-            stdenv = pkgs.clangStdenv;
-          }).stdenv;
-        expected = pkgs.clangStdenv;
-      };
 
       overridePythonAttrs = {
         expr = (applyOverridePythonAttrs package-stub).overridePythonAttrsFlag;