ACPI: video: Remove acpi_video_set_dmi_backlight_type()

acpi_video_set_dmi_backlight_type() is troublesome because it may end
up getting called after other backlight drivers have already called
acpi_video_get_backlight_type() resulting in the other drivers
already being registered even though they should not.

In case of the acpi_video backlight, acpi_video_set_dmi_backlight_type()
actually calls acpi_video_unregister_backlight() since that is often
probed earlier, leading to userspace seeing the acpi_video0 class
device being briefly available, leading to races in userspace where
udev probe-rules try to access the device and it is already gone.

All callers have been fixed to no longer call it, so remove
acpi_video_set_dmi_backlight_type() now.

This means we now also no longer need acpi_video_unregister_backlight()
for the remove acpi_video backlight after it was wrongly registered hack,
so remove that too.

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