ath10k: remove htt rx amsdu clear retry bit hack

With commit 0cfcefef1 ("mac80211: support reporting A-MSDU subframes
individually") there's no need to have the hack to clear the retry bit in
ath10k_htt_rx_amsdu(), mac80211 can handle this properly now.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>