ARM: at91/defconfig: Remove unaffected config option

The commit bf4289cb removed the use of
CONFIG_MTD_NAND_ATMEL_ECC_NONE and CONFIG_MTD_NAND_ATMEL_ECC_HW but the
Kconfig file was forgotten.

This patch remove those inoperative options.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>