mac80211: remove sta->last_ack_signal

This file only feeds a debugfs file that isn't very useful, so remove
it. If necessary, we can add other ways to get this information, for
example in the NL80211_CMD_PROBE_CLIENT response.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/status.c

@@ -863,9 +863,6 @@ void ieee80211_tx_status(struct ieee80211_hw *hw, struct sk_buff *skb)
 				ieee80211_lost_packet(sta, info);
 			}
 		}
-
-		if (acked)
-			sta->last_ack_signal = info->status.ack_signal;
 	}
 
 	rcu_read_unlock();