thermal: core: Drop thermal_zone_device_register()

There are no more users of thermal_zone_device_register(), so drop it
from the core.

Note that thermal_zone_device_register_with_trips() may be renamed to
thermal_zone_device_register() in the future, but only after a grace
period allowing all of the possible work in progress that may be using
the latter to adjust.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>