Remove AppleTalk support again

It seems that neither Windows nor macOS support DDP sockets, but only
the layers on top, while neither Linux nor NetBSD support the layers on
top, but only provide DDP.

It seems that DDP is not meant to be used directly, while Linux and
NetBSD only provide the minimum to use netatalk, which is DDP. ObjFW
would need to use the layers on top on Windows and macOS and implement
those layers itself on Linux and NetBSD. This also means that no code
would be shared between Linux / NetBSD and Windows / macOS, as it's
impossible to have one implementation that works for everything. At this
point, the effort required doesn't make sense.