ath9k: simplify ath_tid_drain

ath_tid_drain is only called when a station entry is being removed, so
there is no point in still tracking BAW state. Remove some unnecessary
code and a bogus TODO comment related to this.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>