rockchip: puma-rk3399: remove unused default ENV_OFFSET for SPI flashes

CONFIG_ENV_OFFSET is set in the defconfig to a different value already
so this isn't used. Let's remove it as to not confuse users.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>