After moving the route control plane code from `net/route.c`, all `rtzone` users ended up being in `net/route_ctl.c`.
Move uma(9) `rtzone` setup/teardown code to `net/route_ctl.c` as well to have everything in a single place.
While here, remove custom initializers from the zone. It was added originally to avoid setup/teardown of costy per-cpu couters. With these counters removed, the only remaining job was avoiding rte mutex setup/teardown. Mutex setup is relatively cheap. Additionally, this mutex will soon disappear. With that in mind, there is no sense in keeping custom zone callbacks.