ath11k: Remove unused function ath11k_htc_restore_tx_skb()

There is no caller in tree, so can remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200918131242.24000-1-yuehaibing@huawei.com