cpuidle: menu governor - remove unused macro STDDEV_THRESH

STDDEV_THRESH was once defined and used in menu governor. But now its no longer
used anywhere. So removing the define.

Signed-off-by: Mohammad Merajul Islam Molla <meraj.enigma@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>