ACPI: PM: Drop unused function and function header

Remove a leftover function header and a static inline stub with no
users from the ACPI header file.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>