Present single conference numbering scheme to the public

[積丹尼 Dan Jacobson <jidanni@jidanni.org>]

Even though true, it is still confusing to the media to
have two competing numbering systems on the same page
$ w3m -dump https://debconf18.debconf.org/ | grep 1[89]|s
DebConf18
The 19th Debian Conference will be held in Hsinchu

So I would move such facts to a history page, and
just present a single numbering scheme to the public.

Most people will otherwise perceive it as an error and dismiss the
conference as not even having enough staff to proofread its own pages.