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

drivers/net/wireless/ath/ath9k/xmit.c: In function 'ath_tx_count_frames':
drivers/net/wireless/ath/ath9k/xmit.c:413:25: warning: variable 'fi' set but not used [-Wunused-but-set-variable]
drivers/net/wireless/ath/ath9k/xmit.c: In function 'ath_tx_complete_aggr':
drivers/net/wireless/ath/ath9k/xmit.c:449:24: warning: variable 'hdr' set but not used [-Wunused-but-set-variable]
drivers/net/wireless/ath/ath9k/xmit.c: In function 'ath_tx_start':
drivers/net/wireless/ath/ath9k/xmit.c:2274:18: warning: variable 'avp' set but not used [-Wunused-but-set-variable]
drivers/net/wireless/ath/ath9k/xmit.c:2269:24: warning: variable 'hdr' set but not used [-Wunused-but-set-variable]

These variables are not used any more
and can be removed.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>