What's the feature?

The logical router policy "nexthop" field has been deprecated in favor of the new "nexthops" field for a very long time, since 21.03: https://github.com/ovn-org/ovn/commit/35b00c7

Why is it needed?

We should change ovn-northd so that it doesn't try to read the deprecated field anymore and only generate SB contents based on the new Logical_router_policy.nexthops field.

Unit and system tests must also be updated.

Who will benefit? 

This simplifies the OVN code base.