sandbox: net: Remove fake-host-hwaddr

Instead of reading a pseudo-rom mac address from the device tree, just use
whatever we get from write_hwaddr. This has the effect of using the mac
address from the environment (or from the device tree, if it is
specified).

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Ramon Fried <rfried.dev@gmail.com>