net: delete redundant function declaration

The implement of function netdev_all_upper_get_next_dev_rcu has been
removed in:
	commit f1170fd4 ("net: Remove all_adj_list and its references")
so delete redundant declaration in header file.

Fixes: f1170fd4

 ("net: Remove all_adj_list and its references")
Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Link: https://lore.kernel.org/r/20211013094702.3931071-1-chenwandun@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>