iwlwifi: rs: cleanup last_txrate_idx

last_txrate_idx isn't used anymore and can be dropped as this info
exists already somewhere else.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>