rt2x00: remove get_tx_stats() mac80211 op

get_tx_stats() will be removed from mac80211.

Compile-tested only.

Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>