thermal/drivers/cpu_cooling: Remove pointless field

The structure cpufreq_cooling_device provides a backpointer to the thermal
device but this one is used for a trace and to unregister. For the trace,
we don't really need this field and the unregister function as the same
pointer passed as parameter. Remove it.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>