mtd: nand: Remove the "nand-disk" LED trigger

This commit removes the "nand-disk" LED trigger from the
NAND code.

A trigger with the same name is already available selecting
LEDS_TRIGGER_MTD. Note that "nand-disk" trigger is being
deprecated in favor of the "mtd" trigger.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>