env: nvram: Let generic env_init() assign default environment

env_nvram_init() assigns default environment if ENV_INVALID, but this is
done in the generic env_init() function, which calls this initializer,
so drop it from here.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>