CI: remove labeler workflow and enable webhook-based labeling

CI: remove labeler workflow and enable webhook-based labeling

Remove the .github/workflows/labeler.yml workflow and enable the "enable_labeler_yml" option in .github/formalities.json to let the webhook bot handle pull request labeling. This avoids running on GitHub Actions, so it is faster and uses fewer resources.

It uses the existing .github/labeler.yml for labels.

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/24256
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>