thermal: Remove thermal_zone_device_update() stub

All users of the function depends on THERMAL, no stub is
needed. Remove it.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20200402142747.8307-9-daniel.lezcano@linaro.org