board: synquacer: developerbox: Don't set gd->env_addr to default_environment

This board sets gd->env_addr to default_environment in board_init(), but
the board has environment in SPI flash according to defconfig. Let the
env API handle environment automatically.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>