thermal: int3403: Delete a check before thermal_zone_device_unregister()

The thermal_zone_device_unregister() function tests whether its argument
is NULL and then returns immediately. Thus the test around the call
is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>