thermal/drivers/cpufreq_cooling: Remove unused list

There is a list with the purpose of grouping the cpufreq cooling
device together as described in the comments but actually it is
unused, the code evolved since 2012 and the list was no longer needed.

Delete the remaining unused list related code.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://lore.kernel.org/r/20210314111333.16551-5-daniel.lezcano@linaro.org