ath9k: remove the noise floor value in the ani struct

common->ani.noise_floor is now only used for a similar redundant debug
message similar to the one that was removed from ath9k_htc in an earlier
patch. Remove it from ath9k as well now.

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