ath10k: remove htc polling for tx completion

Since polling for tx completion is handled whenever target to host
messages are received, removing the unnecessary polling mechanism for
send completion at HTC level.

Reviewed-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>