leds-lp55xx: clean up _remove()

 Replace lp5521/5523_unregister_sysfs() with lp55xx_unregister_sysfs().

 On unloading the driver, running engines should be stopped.
 Use explicit driver function, lp5521/5523_stop_engine().

 Unused functions are removed.

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