metadata: Drop unsupported key 'type'

Fix issue [1] which prevents indexer journal client from consuming the journal.

• [1] https://sentry.softwareheritage.org/share/issue/1d3de3b47c234408889bff5c4f4b0d20/

Related to swh/infra/sysadm-environment#4459 (closed)

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Migrated from D8340 (view on Phabricator)