Remove 'offset' and 'negative_utc'
This only keeps 'offset_bytes' to store the timezone, to match swh-model v5.0.0.
However, this keeps writing 'offset' and 'negative_utc' to the postgresql database, just in case we need to roll back this change. But they are not read anymore.
Test Plan
Will fail because this depends on swh-model!313 (closed), which isn't released yet.
Migrated from D6937 (view on Phabricator)