mlxsw: core_thermal: Call thermal_zone_device_unregister() unconditionally

mlxsw: core_thermal: Call thermal_zone_device_unregister() unconditionally

The function returns immediately if the thermal zone pointer is NULL so
there is no need to check it before calling the function.

Remove the check.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Vadim Pasternak <vadimp@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Link: https://patch.msgid.link/0bd251aa8ce03d3c951983aa6b4300d8205b88a7.1722345311.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>