net: remove support for per driver ndo_busy_poll()

We added generic support for busy polling in NAPI layer in linux-4.5

No network driver uses ndo_busy_poll() anymore, we can get rid
of the pointer in struct net_device_ops, and its use in sk_busy_loop()

Saves NETIF_F_BUSY_POLL features bit.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>