mtd: davinci_nand: remove redundant mtd_device_unregister

mtd_device_unregister is done in nand_release(),
thus no need to call it in nand_davinci_remove().

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