thermal/core: Remove unused functions rebind/unbind exception

The functions thermal_zone_device_rebind_exception and
thermal_zone_device_unbind_exception are not used from anywhere.

Remove that code.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Thara Gopinath <thara.gopinath@linaro.org>
Link: https://lore.kernel.org/r/20201214233811.485669-2-daniel.lezcano@linaro.org