leds-lp55xx: use lp55xx_unregister_leds()

 To unregister led class devices and sysfs attributes,
 LP5521 and LP5523 have each driver function.
 This patch makes both drivers simple using common driver function,
 lp55xx_unregister_leds().

 And some unused variables are removed.

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