ACPI: bus: Drop unused list heads from struct acpi_device

Drop the children and node list heads that have no more users
from struct acpi_device and the code manipulating them from
__acpi_device_add() and acpi_device_del().

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>