thermal: netlink: Drop thermal_notify_tz_trip_add/delete()

Because thermal_notify_tz_trip_add/delete() are never used, drop them
entirely along with the related code.

The addition or removal of trip points is not supported by the thermal
core and is unlikely to be supported in the future, so it is also
unlikely that these functions will ever be needed.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>