thermal: core: Eliminate thermal_zone_trip_down()

Since thermal_zone_set_trip_temp() is now located in the same file
as thermal_trip_crossed(), it can invoke the latter directly without
using the thermal_zone_trip_down() wrapper that has no other users.

Update thermal_zone_set_trip_temp() accordingly and drop
thermal_zone_trip_down().

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://patch.msgid.link/1807510.VLH7GnMWUR@rjwysocki.net