iwlwifi: remove priv->active_rate_basic

This variable is assigned a default value,
but then assigned zero as soon as mac80211
calls a change channel (which will happen
right after the hw is started) and after
that it never changes again.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>