ath9k_hw: remove baseband rfsilent support

When rfkill is enabled, ath9k_hw unnecessarily configured the baseband to
turn off based on GPIO input, however that code was hardcoded to GPIO 0
instead of ah->rfkill_gpio.
Since ath9k uses software rfkill anyway, this code is completely unnecessary
and should be removed in case anything else ever uses GPIO 0.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>