fetchurl: drop stale rewriteURL defaultText

The `rewriteURL` default was changed from `lib.id` to `null`, but the
`defaultText` override still rendered it as `(url: url)` in the manual.
`null` is a primitive the option-doc system renders natively, so just
drop the override.

Assisted-by: Claude Code (claude-opus-4-8)

pkgs/top-level/config.nix

@@ -433,7 +433,6 @@ let
         The intended use is to allow URL rewriting to insert company-internal mirrors, or work around company firewalls and similar network restrictions.
       '';
       default = null;
-      defaultText = literalExpression "(url: url)";
       example = literalExpression ''
         {
           # Use Nix like it's 2024! ;-)