webapp: Remove support for local pairing

webapp: Remove support for local pairing

As a feature only supported by the webapp, and not by git-annex at the
command line, this is by now a very obscure corner of git-annex, and not
one I want to keep maintaining.

It's worth removing it to avoid the security expsure alone. People using
the assistant w/o the webapp probably don't expect it to be listening on
a UDP port for a handrolled protocol, but it was.

The webapp has supported pairing via magic-wormhole since 2016, which
makes a link including between local computers, albeit with the overhead
of tor. That sort of covers the same use case. Of course advanced users
can easily enough add a ssh remote to their repository themselves, using
a hostname on the local network.

Sponsored-by: unqueued