arm: mvebu: Espressobin: Fix setting $fdtfile env

Ensure that 'env default -a' always set correct value to $fdtfile, even
when custom user variable is already stored in non-volatile env storage
(means that env_get("fdtfile") call returns non-NULL value).

As default value is now correctly set like if specified at compile time in
CONFIG_EXTRA_ENV_SETTINGS, there is no need to set $fdtfile explicitly via
env_set("fdtfile", ...) call.

So remove wrong skip based on env_get("fdtfile") and then also unneeded
env_set("fdtfile", ...) call.

Fixes: c4df0f6f ("arm: mvebu: Espressobin: Set default value for $fdtfile env variable")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>