ACPI / scan: remove unused LIST_HEAD(acpi_device_list)

The acpi_device_list list is not used, so removed it.

[rjw: Changelog]
Signed-off-by: Haicheng Li <haicheng.li@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>