ath9k_hw: remove the old ANI implementation

It was found to be buggy on a variety of chipsets from AR913x to AR928x.
The new version (which was introduced along with AR93xx support) is more
reliable in preventing connectivity dropouts and also fixes MIB interrupt
storm issues.

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