mac80211: rc80211_minstrel: remove variance / stddev calculation

When there are few packets (e.g. for sampling attempts), the exponentially
weighted variance is usually vastly overestimated, making the resulting data
essentially useless. As far as I know, there has not been any practical use
for this, so let's not waste any cycles on it.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>