ACPI / PM: Convert acpi_dev_pm_detach() into a static function

The ->detach() callback for the PM domain has now been fully adopted,
thus there no users left of the acpi_dev_pm_detach() API. This allow us
to convert it into a static function.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>