mac80211: remove get_tx_stats() driver op

get_tx_stats() driver operation is not currently used anywhere in mac80211
and there are no plans to use it in the not-so-near future. So it can go
without anyone missing it.

Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>