cpuidle: Remove manual selection of the multiple driver support

Like the coupled idle state, it is not up to the user to set this option
but the driver to select it.

Remove the interactive selection of this option.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>