virtio_net: remove custom busy_poll

Generic NAPI busy polling allows us to remove custom implementations
found in drivers.

It is possible further optimization could be done by testing
napi_complete_done() return value.

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