ath9k: Nuke unneccesary helper function to see if aggr is active

IEEE80211_TX_CTL_AMPDU in tx control flags should be enough
to confirm BA is negotiated for that tid.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>