security: remove security_settime

security_settime was a wrapper around security_settime64. There are no more
users of it. Therefore it can be removed. It was removed in:
commit 4eb1bca1 ("time: Use do_settimeofday64() internally")

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Signed-off-by: James Morris <james.morris@microsoft.com>