thermal: core: Drop the .set_trip_hyst() thermal zone operation

None of the users of the thermal core provides a .set_trip_hyst()
thermal zone operation, so drop that callback from struct
thermal_zone_device_ops and update trip_point_hyst_store()
accordingly.

No functional impact.

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