iwlwifi: remove MFP Kconfig option

Remove the Kconfig option CONFIG_IWLWIFI_EXPERIMENTAL_MFP,
if the firmware doesn't support MFP then the user shouldn't
have the option to enable it as it won't work correctly.

Signed-off-by: Assaf Krauss <assaf.krauss@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>