ath9k: remove unnecessary IEEE80211_TX_CTL_NO_ACK checks

We check for this condition early on in our mac80211 get_rate()
callback ath_get_rate(), so remove this check later down the path.

Cc: Derek Smithies <derek@indranet.co.nz>
Cc: Chittajit Mitra <Chittajit.Mitra@Atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>