net: kill netif_copy_real_num_queues()

vlan was the only user of netif_copy_real_num_queues(),
but it no longer calls it after
commit 4af429d2 ("vlan: lockless transmit path").
So we can just remove it.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>