ACPI: thermal: Do not attach private data to ACPI handles

The ACPI thermal driver uses acpi_bus_attach_private_data() to attach
the thermal zone object to the ACPI handle of the thermal zone and
acpi_bus_detach_private_data() to clean that up, but it never uses
acpi_bus_get_private_data() to retrieve that object.

Drop the unneded acpi_bus_attach_private_data() and
acpi_bus_detach_private_data() calls from the ACPI thermal driver and
clean up the related code.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Zhang Rui <rui.zhang@intel.com>