ath9k: Remove a few unused functions

ATH9K_ANT_VARIABLE is the default diversity control used.
Consequently ath9k_hw_decrease_chain_power() does nothing.
ath9k_hw_setantennaswitch() is unused too.

Also, gbeacon_rate is unused.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>