releases: Remove broken "todo" links

We haven't had any "todo.asc" files since 11.1. Don't provide links
to them, even commented out.

In recent releases we've been using a wiki page for tracking issues,
but I don't think it's particularly useful to include that link here;
during the release cycle developers see that URL at least once a week
and after the release it's no longer relevant.

Inspired by: Graham Perrin