ACPI / PM: Remove stale function header

acpi_dev_pm_get_node() isn't used or implemented, so remove it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>