ACPI / thermal: Remove the unused lock of struct acpi_thermal

The acpi_thermal->lock now just is initialized when a thermal zone
device is added and destroyed when the thermal zone is removed.
It is never used in any other places, so remove it.

Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>