ath9k: remove WDS code

The ability to reach this code was hidden behind
CONFIG_WIRELESS_WDS, which was just removed. Clean
up the driver accordingly.

Link: https://lore.kernel.org/r/20201109105103.e1d48ee20b0a.I89f8af0d50a02ce16a922fa790d6c1908f31a496@changeid


Acked-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>