b43legacy: 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.daa66065cc71.Ie6280e1606fa9aa892b2a342a62aabd42e241f9c@changeid


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