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

env_flash_init() (both implementations) 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>