nexthop: Remove in-kernel route notifications when nexthop changes

Remove in-kernel route notifications when the configuration of their
nexthop changes.

These notifications are unnecessary because the route still uses the
same nexthop ID. A separate notification for the nexthop change itself
is now sent in the nexthop notification chain.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>