igb: remove LRO Kconfig option

To match ixgbe GRO functionality, we remove the LRO (which is now GRO)
Kconfig option which enables GRO by default.

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