Remove link to Windows installer in instruction generator

[osirisinferi]

(A previous issue raising this at #812 was closed while the problem kept existing.)

See https://certbot.eff.org/instructions?ws=webproduct&os=windows. It still links to https://github.com/certbot/certbot/releases/latest/download/certbot-beta-installer-win_amd64_signed.exe which is broken since no new Windows installer are released since 2.10.0 any longer.

Also, perhaps that whole Windows page needs an overhaul instead of just that single line of a warning at the top.

We often get complaints or other threads about this on the Let's Encrypt Community such as https://community.letsencrypt.org/t/certbot-windows-installer-link-is-broken-no-latest-release/233686.

[David263]

I'd like to reinforce that it can be confusing to maintain a website section that contains a notice that support for Windows has been discontinued, but then includes the now-obsolete information about that support. If you wish to keep the obsolete information for historical purposes, I suggest creating a historical archive and replacing the information with a link to a page in that historical archive, along with a prominent notice that the information is obsolete.