ixgb: Remove ixgb driver

There are likely no users of this driver as the hardware has been
discontinued since 2010. Remove the driver and all references to it
in documentation.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>