thermal: core: Drop unused bind/unbind functions and callbacks

There are no more callers of thermal_zone_bind_cooling_device() and
thermal_zone_unbind_cooling_device(), so drop them along with all of
the corresponding headers, code and documentation.

Moreover, because the .bind() and .unbind() thermal zone callbacks would
only be used when the above functions, respectively, were called, drop
them as well along with all of the code related to them.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Huisong Li <lihuisong@huawei.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://patch.msgid.link/4251116.1IzOArtZ34@rjwysocki.net