remove scale: from .zattrs

[mtbc]

{
    "multiscales": [
        {
            "datasets": [
                {
                    "path": "0",
                    "scale": 1.0
                },
                {
                    "path": "1",
                    "scale": 0.5
                },
                {
                    "path": "2",
                    "scale": 0.25
                },
                {
                    "path": "3",
                    "scale": 0.125
                },
                {
                    "path": "4",
                    "scale": 0.0625
                },
                {
                    "path": "5",
                    "scale": 0.03125
                }
            ],
            "version": "0.1"
        }
    ]
}

All the "scale" keys should be omitted from the metadata.

However, unit tests should at least check that resolutions start with the full-size then work downward.