thermal/of: Remove of_thermal_is_trip_valid()

There is no benefit with the of_thermal_is_trip_valid() function as it
does the check the thermal_zone_get_trip() is already doing for the
sake of getting the trip point.

As all the calls have been replaced by thermal_zone_get_trip(), there
is no more users of of_thermal_is_trip_valid().

Remove the function.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20221003092602.1323944-18-daniel.lezcano@linaro.org