thermal: trip: Drop a redundant check from thermal_zone_set_trip()

After recent changes in the thermal framework, a trip points array is
required for registering a thermal zone that is not tripless, so the
tz->trips pointer in thermal_zone_set_trip() is never NULL and the
check involving it is redundant.  Drop that check.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>