MAINTAINERS: remove ath5k mailing list

The list is in the process of closing.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Nick Kossifidis <mickflemm@gmail.com>
Cc: "Luis R. Rodriguez" <mcgrof@do-not-panic.com>
Cc: linux-wireless@vger.kernel.org
Cc: "Michael Renzmann" <mrenzmann@madwifi-project.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>