thermal: core: Remove unused functions in power actor section

Since the Intelligent Power Allocation (IPA) uses different way to get
minimum and maximum power for a given cooling device, the helper functions
are not needed. There is no other code which uses them, so remove the
helper functions.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201015112441.4056-4-lukasz.luba@arm.com