board/BuR: drop ETH-support in SPL-Stage

During very early prototype-phase we did boot the AM335x boards
initially from CPSW-EMAC.

Now we don't need this feature anymore.

So we drop it to save MLO-space and having therefore a more quickly
boot.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Reviewed-by: Tom Rini <trini@konsulko.com>