pgstorage: Drop unnecessary indirection from reading origin_visit
It's a missing left-over from the "make the origin-visit immutable" migration.
Kudos to the previous diff [1] which made me raise an eyebrow and then realized the equivalent method in the cassandra backend was dead code (and it is here as well)
Test Plan
tox is still happy
Migrated from D3598 (view on Phabricator)