arm/km: remove unneeded define

CONFIG_ENV_SIZE for NAND was later in this file overwritten
because we have the environment in i2c eeprom, so remove
this define.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Heiko Schocher <hs@denx.de>