Clean up entries in {origin_intrinsic,revision}_metadata with no metadata

For now, there's ~90% rows whose dictionary has a single @context key.

We should delete them.

When done, the following two queries should both return 0:

softwareheritage-indexer=> select count(*) from revision_intrinsic_metadata where metadata = '{"@context": "https://doi.org/10.5063/schema/codemeta-2.0"}'::jsonb;
 count  
--------
 742428
(1 row)

softwareheritage-indexer=> select count(*) from origin_intrinsic_metadata where metadata = '{"@context": "https://doi.org/10.5063/schema/codemeta-2.0"}'::jsonb;
 count  
--------
 369096
(1 row)

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Migrated from T1549 (view on Phabricator)