nand: Remove CONFIG_MTD_NAND_VERIFY_WRITE

The CONFIG_MTD_NAND_VERIFY_WRITE has been removed from Linux for some
time and a more generic method of NAND verification now exists in U-Boot.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Tested-by: Heiko Schocher <hs@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>