iwlwifi: remove 5ghz_disable option

5ghz_disable has no effect any longer, that was changed during
refactoring of EEPROM reading/parsing. Remove it, wpa_supplicant
allow now to specify frequencies, on which device will operate.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>