ath11k: drop tx_info from ath11k_sta

We will start using ieee80211_tx_status_ext() so we do not need to track
tx rates inside a struct ieee80211_tx_info. It is currently not possible
to populate that struct with HE rate info anyhow.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>