mailaddr(7): wave goodbye

The information here is somewhere between ancient to obsolete.
It refers to a time in the internet's history when manual routing
was still useful, talks about UUCP as if its modern, and refers
to documents which I had trouble tracking down.

It seems unlikely that a manual page in this form would be useful, so
just remove it.

Reviewed By: imp, tsoome, bdrewery(?)
Differential Revision: https://reviews.freebsd.org/D12924