platform/x86: huawei-wmi: Remove unnecessary battery mutex

battery_lock mutex is never used and not needed.

Fixes: 355a070b ("platform/x86: huawei-wmi: Add battery charging thresholds")
Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>