libxml2Python: Remove updateScript

`update.nix` considers this a different package from `libxml2` based on `meta.position`.
As a result `update.nix` in commit mode would crash when it tried to cherry pick `libxml2Python` update after `libxml2` commit has already been picked.

    nix-shell maintainers/scripts/update.nix --arg predicate '(path: pkg: builtins.match "libxml.+" pkg.name != null)' --argstr keep-going true --argstr commit true

Perhaps we should consider synchronizing all the worktrees after each commit so that conflicting updates would not happen in different worktrees but removing the update script is easier.

pkgs/top-level/all-packages.nix

@@ -23207,7 +23207,8 @@ with pkgs;
   in pkgs.buildEnv { # slightly hacky
     name = "libxml2+py-${res.libxml2.version}";
     paths = with libxml2; [ dev bin py ];
-    inherit (libxml2) passthru;
+    # Avoid update.nix conflicts with libxml2.
+    passthru = builtins.removeAttrs libxml2.passthru [ "updateScript" ];
     # the hook to find catalogs is hidden by buildEnv
     postBuild = ''
       mkdir "$out/nix-support"