ipv6: Stop sending in-kernel notifications for each nexthop

Both listeners - mlxsw and netdevsim - of IPv6 FIB notifications are now
ready to handle IPv6 multipath notifications.

Therefore, stop ignoring such notifications in both drivers and stop
sending notification for each added / deleted nexthop.

v2:
* Remove 'multipath_rt' from 'struct fib6_entry_notifier_info'

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>