net: remove exports for netdev_name_node_alt_create() and destroy

netdev_name_node_alt_create() and netdev_name_node_alt_destroy()
are only called by rtnetlink, so no need for exports.

Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20220310223952.558779-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>