thermal: core: Drop the .throttle() governor callback

Since all of the governors in the tree have been switched over to using
the new callbacks, either .trip_crossed() or .manage(), the .throttle()
governor callback is not used any more, so drop it.

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>