EmbeddedPkg: remove Lan91x network controller driver

EmbeddedPkg: remove Lan91x network controller driver

The Lan91x network controller driver does not implement the UEFI driver
model, and is not a suitable example for new drivers to be based on.
Also, it is currently only used on a limited set of ARM development
platforms.

Due to this, it has been moved into the edk2-platforms repository,
alongside its remaining users, which have been updated to refer to it in
its new location. So drop this version from EmbeddedPkg.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>