[Tails-dev] Redundant alt

Hello, useless alt with redundant informations was recently introduce in
three pages.

I made a branch to fix that. Please review.

Repo: https://git-tails.immerda.ch/xin/tails/
Branch: alt
Last Commit: ddc6167b8f551811d7235fe97348c930830d747e

Cheers.
xin