ath10k: remove unused pdev_set_channel command

This command is not used anymore and most firmware
revisions do not seem to handle it well. Channel
switching is done via vdev restarting.

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