igb: removed unused tx/rx total bytes/packets from adapter struct

This patch removes unused variables total_tx_bytes, total_tx_packets,
total_rx_bytes, and total_rx_packets from the adapter struct.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>