ath9k_hw: remove antenna configuration eeprom ops and variables

AR9280 based hardware with 3 antennas and slow antenna diversity has
not been seen in the wild and ath9k does not support that form of
antenna diversity, so remove the EEPROM ops for it.
These EEPROM ops are currently only used for setting the
AR_PHY_SWITCH_COM register, which is being done in the EEPROM specific
file already.

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