backlight: aat2870-backlight: Remove struct backlight.check_fb

backlight: aat2870-backlight: Remove struct backlight.check_fb

The driver's implementation of check_fb always returns true, which
is the default if no implementation has been set. So remove the code
from the driver.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20240305162425.23845-6-tzimmermann@suse.de
Signed-off-by: Lee Jones <lee@kernel.org>