net: remove zap_completion_queue

netpoll does an interesting work in zap_completion_queue(), but this was
before we did skb orphaning before delivering packets to device.

It now makes sense to add a test in dev_kfree_skb_irq() to not queue a
skb if already orphaned, and to remove netpoll zap_completion_queue() as
a bonus.

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