ath9k: remove rssi/antenna information from recv debug stats

The way this is implemented (simply storing the last value) is absolutely
worthless for debugging anything, and the same information is also available
through the MAC sample feature, so there's no point in keeping this around.

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