Documentation: networking: ixgb: Remove reference to IXGB_NAPI

NAPI is enabled by default and IXGB_NAPI was removed since
commit 6d37ab28 ("ixgb: make NAPI the only option and the default")
Update the doc accordingly.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>