mtd: nand: jz4740: remove custom 'erased check' implementation

The jz4740 driver is manually checking for 'erased pages' while
correcting ECC bytes.
This logic can now done by the core infrastructure, and can thus be removed
from this driver.

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