ath10k: snoc: remove set but not used variable 'ar_snoc'

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

drivers/net/wireless/ath/ath10k/snoc.c: In function 'ath10k_snoc_tx_pipe_cleanup':
drivers/net/wireless/ath/ath10k/snoc.c:681:22: warning:
 variable 'ar_snoc' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>