rgw: remove bucket API returns NoSuchKey than NoSuchBucket

rgw: remove bucket API returns NoSuchKey than NoSuchBucket

Remove bucket API returns NoSuchKey but NoSuchBucket is appropriate in this case.

Code path:
RGWRadosStore::get_bucket
-> RGWRadosBucket::get_bucket_info
-> RGWBucketCtl::read_bucket_info
-> RGWBucketCtl::read_bucket_entrypoint_info
-> RGWSI_Bucket_SObj::read_bucket_entrypoint_info
-> RGWSI_MetaBackend_SObj::get_entry
-> rgw_get_system_obj
-> RGWSI_SysObj::Obj::ROp::stat
-> RGWSI_SysObj_Core::stat # return -ENOENT here.

[1]: https://docs.ceph.com/en/pacific/radosgw/adminops/#remove-bucket

Fixes: https://tracker.ceph.com/issues/53731
Signed-off-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>