wifi: ath5k: remove unused ath5k_eeprom_info::ee_antenna

clang-struct [1] found that ee_antenna in struct ath5k_eeprom_info is
unused. The commit 1048643e

 ("ath5k: Clean up eeprom
parsing and add missing calibration data") added it, but did not use it
in any way. Neither, there is a later user.

So remove that unused member.

[1] https://github.com/jirislaby/clang-struct

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Felix Fietkau <nbd@openwrt.org>
Cc: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Kalle Valo <kvalo@kernel.org>
Cc: linux-wireless@vger.kernel.org
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20231205111515.21470-1-jirislaby@kernel.org