network/dhcp-pd: do not remove unreachable route when reconfiguring non-upstream...

network/dhcp-pd: do not remove unreachable route when reconfiguring non-upstream interface

Unreachable routes are not owned by any interfaces, and its ifindex is
zero. Previously, if a non-upstream interface is reconfigured, all routes
including unreachable routes configured by the upstream interface are
removed.

This makes unreachable routes are always handled by the upstream interface,
and only removed when the delegated prefixes are changed or lost.