Documentation: networking: ixgb: Remove reference to IXGB_NAPI

NAPI is enabled by default and IXGB_NAPI was removed since
commit 6d37ab282e24 ("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>

Documentation/networking/ixgb.txt

@@ -323,8 +323,7 @@ Additional Configurations
   NAPI
   ----
 
-  NAPI (Rx polling mode) is supported in the ixgb driver.  NAPI is enabled
-  or disabled based on the configuration of the kernel.  see CONFIG_IXGB_NAPI
+  NAPI (Rx polling mode) is supported in the ixgb driver.
 
   See www.cyberus.ca/~hadi/usenix-paper.tgz for more information on NAPI.