ath9k: remove set but not used variable 'acq'

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

drivers/net/wireless/ath/ath9k/recv.c: In function 'ath_rx_count_airtime':
drivers/net/wireless/ath/ath9k/recv.c:1010:18: warning:
 variable 'acq' set but not used [-Wunused-but-set-variable]

It's not used after 89cea749 ("ath9k: Switch to mac80211 TXQ scheduling
and airtime APIs"). Also remove related variables.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>