thermal: trip: Drop thermal_zone_get_trip()

There are no more callers of thermal_zone_get_trip() in the tree, so
drop it.

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/2220301.Mh6RI2rZIc@rjwysocki.net