backlight: Remove notifier

backlight_register_notifier and backlight_unregister_notifier have
been unused since commit 6cb634d0 ("ACPI: video: Remove code to
unregister acpi_video backlight when a native backlight registers")

With those not being called, it means that the backlight_notifier
list is always empty.

Remove the functions, the list itself and the enum used in the
notifications.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20240919232758.639925-1-linux@treblig.org


Signed-off-by: Lee Jones <lee@kernel.org>