ath9k_hw: remove aggregation protection mode

when aggregation protection mode is enabled the hardware needs
to send RTS/CTS for each HT frame. Currently its disabled so
remove the unused call backs.

Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>