network: do not remove reject type routes more than once

network: do not remove reject type routes more than once

Usually, removing non-existing addresses, routes, and etc, are safe.
However, when multiple interfaces lost their carriers simultaneously,
then manager_drop_routes() and manager_drop_nexthop() are called multiple
times. If a route with a blackhole nexthop is removed in that process,
the later removal requests of the route fail with -EINVAL, rathar
than -ESRCH, as the corresponding nexthop does not exist anymore.

So, let's not remove routes which managed by Manager more than once.