thermal: intel: intel_soc_dts_iosf: Remove redundant check

Remove the redundant check in remove_dts_thermal_zone() because all of
its existing callers pass a valid pointer as the argument.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>