igb: remove adaptive IFS from driver

Adaptive IFS support has been included in the igb driver since its
initial release, but it is not a feature on any igb NICs. This patch
removes it from the driver.

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