ath9k_hw: remove defunct ad-hoc mode ATIM window handling code

The hardware does not have support for ATIM processing, and the driver
does not set up ah->atim_window anywhere. Additionally, the code can
clobber the timer used by P2P powersave.

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