leds-lp55xx: fix problem on removing LED attributes

 LP55XX common device attributes, 'led_current' and 'max_current' are created
 while loading the driver.
 Those are LED device attributes which are removed automatically on releasing
 led class devices - led_classdev_unregister().
 Therefore, this duplicate code should be removed.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>