The silent environment callback function does not update the silent
flag during silent env set or unset. That is because of duplicated
CONFIG keyword at preprocessor condition in silent environment
callback function and cause silent env callback unable to work.

This patch is to remove the duplicated CONFIG keywork in silent
environment callback function.

Signed-off-by: Wilson Lee <wilson.lee@ni.com>
Cc: Keng Soon Cheah <keng.soon.cheah@ni.com>
Cc: Chen Yee Chew <chen.yee.chew@ni.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>