ath5k: remove get_tx_stats() mac80211 op

get_tx_stats() will be removed from mac80211.

Compile-tested only.

Cc: Jiri Slaby <jirislaby@gmail.com>
Cc: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>