board: freescale: t208xrdb: remove the legacy non-DM_ETH code

Both DM_ETH and DM_PCI are enabled for the T2080RDB board. Remove the
board_eth_init() callback and the non-DM_ETH code paths since they are not
needed anymore.

Signed-off-by: Camelia Groza <camelia.groza@nxp.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>