mac80211: remove set but not used variable 'old'

Fixes gcc '-Wunused-but-set-variable' warning:

net/mac80211/key.c: In function 'ieee80211_set_tx_key':
net/mac80211/key.c:271:24: warning:
 variable 'old' set but not used [-Wunused-but-set-variable]

It is not used since introduction in
commit 96fc6efb ("mac80211: IEEE 802.11 Extended Key ID support")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/key.c

@@ -268,11 +268,9 @@ int ieee80211_set_tx_key(struct ieee80211_key *key)
 {
 	struct sta_info *sta = key->sta;
 	struct ieee80211_local *local = key->local;
-	struct ieee80211_key *old;
 
 	assert_key_lock(local);
 
-	old = key_mtx_dereference(local, sta->ptk[sta->ptk_idx]);
 	sta->ptk_idx = key->conf.keyidx;
 	ieee80211_check_fast_xmit(sta);