ath9k: Remove unused BMISS processing

The various variables tracking bmiss interrupts
are not really used anywhere except in a debug
message. Remove them since they have no functional
purpose.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>