Remove buildtime_bindgen from bundled feature

[rkusa]

The README mentioned that you get pregenerated bindings if the bundled feature is used:

If you use the bundled features, you will get pregenerated bindings for the bundled version of DuckDB. If you want to run bindgen at buildtime to produce your own bindings, use the buildtime_bindgen Cargo feature.

However, 2a50047 added the buildtime_bindgen to the bundled feature. So this statement doesn't hold true anymore.

This PR removes buildtime_bindgen from the bundled feature again, just in case including it wasn't on purpose. Otherwise, I'd like to suggest a README update instead 😊.

[wangfenjin]

Thanks for your contribution!