mac80211: remove tx status call to ieee80211_sta_register_airtime

All drivers using airtime fairness are calling ieee80211_sta_register_airtime
directly, now they must. Document this as well.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20200908123702.88454-8-nbd@nbd.name


[johannes: update the documentation to suit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>