remove storage:refs from assets with data-download

[alambare]

Example of a bug with the storage:refs asset property when using the data-download extension:

{
                "zarr": {
                    "title": "Timeseries Zarr store for METEO station Oberwölz_NLV",
                    "href": "https://hda-download-staging.lumi.data.destination-earth.eu/data/internal_fdp/STAT.ICPDR.DAT.DANUBEHIS/AT123158_METEO/zarr",
                    "type": "application/x-zarr",
                    "roles": [
                        "auxiliary"
                    ],
                    "updated": "2025-07-05T10:10:29+00:00",
                    "file:size": 4242,
                    "storage:refs": [
                        "dedl-central-fdp"
                    ],
                    "alternate": {
                        "origin": {
                            "title": "Timeseries Zarr store for METEO station Oberwölz_NLV",
                            "href": "s3://fresh-datapool/DANUBEHIS/zarr/AT123158_METEO.zarr",
                            "type": "application/x-zarr",
                            "roles": [
                                "auxiliary"
                            ],
                            "updated": "2025-07-05T10:10:29+00:00",
                            "file:size": 4242,
                            "storage:refs": [
                                "dedl-central-fdp"
                            ]
                        }
                    }
                }

The HREF is overriden by the extension and is not anymore a S3 type but a HTTP URL. The storage:refs needs to be removed.
However, we must keep it for the alternate/origin section as well as the storage:schemes in the item properties.