platform/loongarch: laptop: Stop setting acpi_device_class()

platform/loongarch: laptop: Stop setting acpi_device_class()

The driver populates acpi_device_class() which is never read afterward,
so make it stop doing that and drop the symbol defined specifically for
this purpose.

No intentional functional impact.

This change will facilitate the removal of "device_class" from "struct
acpi_device_pnp" in the future.

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