Remove Documentation -> 2019.www.torproject.org from the menu

We have too many users visiting the old Tor docs and getting lost or using old instructions. That is because of search engines, but also because we say "Documentation," but most of the docs there live in the Support portal and Community portal.

As the main team plan was to replace it with a new developer portal (See: tpo/web/dev), I would like to track the minimal work we need to do to remove that website from the header once for all.