ospfd: Use router_id what Zebra has if we remove a static router_id

ospfd: Use router_id what Zebra has if we remove a static router_id

If we set router-id, e.g. `router-id x.x.x.x`, then we have:

```
pc.donatas.net# show ip ospf | include Router ID
 OSPF Routing Process, Router ID: x.x.x.x
```

But once we remove it (`no router-id x.x.x.x`), the old router-id remains.

This is kinda OK, but to be consistent with OSPFv3 we should use what Zebra
already has, instead of retaining the old one.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>