Remove the code to unregister acpi_video backlight devices when
a native backlight device gets registered later.

Now that the acpi_video backlight device registration is a separate step
which runs later, after the drm/kms driver is done setting up its own
native backlight device, it is no longer necessary to monitor for a
native (BACKLIGHT_RAW) device showing up later and to then unregister
the acpi_video backlight device(s).

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>