No change in behavior. All existing usages of `replicables_for_geo_node`
already apply `primary_key_in` or `id_in` scopes, so this is only a
refactor.

This refactor will allow us to apply the scope to a subquery within
`LfsObject.replicables_for_geo_node` to improve performance.

This refactor will also tend to ensure that `replicables_for_geo_node`
is not accidentally used without restricting the scope.