efi_loader: Drop code that doesn't work with driver model

This code should never have been added as it builds a new feature on top
of legacy code. This has already been improved with the dependency on BLK.

Add a dependency on DM_ETH also, to avoid needing to deal with this old
code.

Boards which want EFI_LOADER should migrate to driver model first.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>