staging: comedi: cb_pcidas64: remove useless function

Fix the following gcc warning:

drivers/staging/comedi/drivers/cb_pcidas64.c:232:19: warning: unused
function 'analog_trig_low_threshold_bits' [-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Link: https://lore.kernel.org/r/1617949425-105420-1-git-send-email-jiapeng.chong@linux.alibaba.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>