drivers/net/igb: remove dead code (function 'igb_read_pci_cfg')

Fix this warning:

  drivers/net/igb/e1000_mac.c:54: warning: 'igb_read_pci_cfg' defined but not used

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>