ARM: at91/defconfig: Remove unaffected config option

The commit bf4289cba02b8cf770ecd7959ca70839f0dd9d3c 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>