rgw: S3 Delete Bucket Policy should return 204 on success

rgw: S3 Delete Bucket Policy should return 204 on success

Currently, RGW returns a 200 on a successful DELETE on a bucket policy but
the S3 API expects a 204:

    https://docs.aws.amazon.com/AmazonS3/latest/API/API_DeleteBucketPolicy.html

The sample response in the example above is a 204.

This patch checks the op_ret in `RGWDeleteBucketPolicy::send_response()`
and on a success we turn it to a 204 (or STATUS_NO_CONTENT).

Fixes: https://tracker.ceph.com/issues/69539
Signed-off-by: Simon Jürgensmeyer <simon.juergensmeyer@hetzner-cloud.de>