ACPI: delete unused acpi_bus_drivers_lock

acpi_bus_drivers is protected by acpi_device_lock
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/scan.c

@@ -471,7 +471,6 @@ static int acpi_bus_get_perf_flags(struct acpi_device *device)
    -------------------------------------------------------------------------- */
 
 static LIST_HEAD(acpi_bus_drivers);
-static DECLARE_MUTEX(acpi_bus_drivers_lock);
 
 /**
  * acpi_bus_match - match device IDs to driver's supported IDs