ath9k: remove tid->paused flag

There are some corner cases where the driver could get stuck with a full
tid queue that is paused, leading to a software tx queue hang.

Since the tx queueing rework, pausing per-tid queues on aggregation
session setup is no longer necessary. The driver will assign sequence
numbers to buffered frames when a new session is established, in order
to get the correct starting sequence number.

mac80211 prevents new frames from entering the queue during setup.

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