storage.db: Drop db.origin_visit_upsert behavior
The initial desired behavior was to allow creation of origin-visit if they
already had their id set. This is the what's needed for the replayer to actually
But somehow, this left the possibility to update the origin-visit...
This commit fixes it by dropping conflictual origin-visits if any.
In effect, we can no longer overwrite origin-visits (pg-storage wise).
Related to T2310