mtd: onenand: remove redundant mtd_device_unregister before onenand_release

onenand_release() will call mtd_device_unregister(), thus remove the redundant
mtd_device_unregister() call before onenand_release().

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>