Update Default Value for Metadata_Valid

Change the default value from true to None. This is limiting us on the front end in what we can properly return for calculations on sets of metadata. Thanks!

https://code.usgs.gov/ghsc/geomag/geomag-algorithms/-/blob/master/geomagio/api/secure/metadata.py#L43

assigned to @pcain

mentioned in merge request !112 (closed)

Opting to remove this parameter altogether and merge its purpose into status. New status to support this will be "rejected"

mentioned in merge request !113 (merged)

mentioned in commit a9fdfa62

closed with merge request !113 (merged)