deskutils/gnome-utils: Temporarily disable deskutils/gnome-todo

The port deskutils/gnome-todo has failed to build for months, which
makes deskutils/gnome-utils and eventually x11/gnome3 packages missing.
The build potentially needs a non-trivial fix, so temporarily remove
deskutils/gnome-todo from build while the issue is being investigated.

PR: 257667
Reviewed by: tcberner (gnome)