thermal: Remove netlink support

There are no users of netlink messages for thermal inside the kernel.
Remove the code and adjust the documentation.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/8ff02cf62186c7a54fff325fad40a2e9ca3affa6.1571656014.git.amit.kucheria@linaro.org