thermal/drivers/clock_cooling: Remove clock_cooling code

clock_cooling has no in-kernel users. It has never found any use in
drivers as far as I can tell.

Remove the code.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/aa5d5ac2589cf7b14ece882130731b4a916849a6.1593619943.git.amit.kucheria@linaro.org