wifi: libertas: main: remove unused functions

lbs_data_rate_to_fw_index(), lbs_enter_auto_deep_sleep() and
lbs_exit_auto_deep_sleep() last use was removed in 2010 by
commit e86dc1ca ("Libertas: cfg80211 support").

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20250121013300.433538-2-linux@treblig.org