eth: igc: remove unused embedded struct net_device

struct net_device poll_dev in struct igc_q_vector was added
in one of the initial commits, but never used.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>