backlight: adp8870: Remove unnecessary MODULE_ALIAS()

The driver has a I2C device id table that is used to create the modaliases
and also "adp8870-backlight" is not a supported I2C id, so it's never used.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>