ath10k: remove unused variable ar_pci

Trival fix to remove unused variable ar_pci in ath10k_pci_tx_pipe_cleanup
when building with W=1:
drivers/net/wireless/ath/ath10k/pci.c:1696:21: warning: variable
'ar_pci' set but not used [-Wunused-but-set-variable]

Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>