mtd: nand: jz4740: kill the ->ecc_layout field

->ecc_layout is not used by any board file. Kill this field to avoid any
confusion. New boards are encouraged to use the default ECC layout defined
in NAND core.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>