thermal/of: Remove of_thermal_get_crit_temp()

The generic version of of_thermal_get_crit_temp() can be used. Let's
remove this ops which is pointless.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20221003092602.1323944-20-daniel.lezcano@linaro.org