board: ti: dra7-evm: remove net platform code

The DRA7 has DM_ETH and OF_CONTROL enabled, so remove networking platform
code.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>