iwlwifi: remove reference to non-existent documentation

The external iwlwifi driver comes with a README file that is
referenced by the Kconfig. This README is not present in the
driver included in the kernel. Remove references to this
documentation.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>