staging: remove ath6kl

ath6kl is now in drivers/net/wireless/ath so the staging driver
is not supported anymore and should be removed.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>