Website still links to old email list archives.

[bangerth]

The deal.II website, under "Mailing lists" at https://dealii.org/mail.html, still links to archives from our pre-Google era. These archives are no longer accessible (i.e., the link is dead) and it's questionable whether they are still useful to begin with. We should just remove the link/the whole box.

If we're removing the whole box on that page, there is no more content to https://dealii.org/mail.html and we may as well directly link to the google forum from the drop-down menu.