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>