e1000: e1000_adapter->polling_netdev is useless

Commit bea3348e


"[NET]: Make NAPI polling independent of struct net_device objects."
made NAPI polling to be independent of net_device.
So e1000_adapter->polling_netdev is no longer used.
Kill it.

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>