thermal/core: Remove thermal_zone_set_trips()

Since no callers of thermal_zone_set_trips() are left, remove the function.
Document __thermal_zone_set_trips() instead. Explicitly state that the
thermal zone lock must be held when calling the function, and that the
pointer to the thermal zone must be valid.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>