MEDIUM: proxy: remove obsolete "monitor-net"

MEDIUM: proxy: remove obsolete "monitor-net"

As discussed here during 2.1-dev, "monitor-net" is totally obsolete:

   https://www.mail-archive.com/haproxy@formilux.org/msg35204.html

It's fundamentally incompatible with usage of SSL, and imposes the
presence of file descriptors with hard-coded syscalls directly in the
generic accept path.

It's very unlikely that anyone has used it in the last 10 years for
anything beyond testing. In the worst case if anyone would depend
on it, replacing it with "http-request return status 200 if ..." and
"mode http" would certainly do the trick.

The keyword is still detected as special by the config parser to help
users update their configurations appropriately.