ath9k: remove the pending frames ath_txq_schedule workaround

This workaround called ath_txq_schedule whenever there were still pending
frames for a queue, but the queue depth was zero. Because of its its high
false positive probability (e.g. with paused TIDs) and because it is in
the way of other pending work (AP powersave fixes), it is better to remove
this code entirely.

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