appletalk: Remove deadcode

alloc_ltalkdev in net/appletalk/dev.c is dead since
commit 00f3696f ("net: appletalk: remove cops support")

Removing it (and it's helper) leaves dev.c and if_ltalk.h empty;
remove them and the Makefile entry.

tun.c was including that if_ltalk.h but actually wanted
the uapi version for LTALK_ALEN, fix up the path.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>