main-window: Hide the link button

There's definitely something going wrong with linking contacts, as it
forgets any link between app restarts. Since we really don't want to
expose something that's completely broken to our users, hide the button
that allows the linking. It's a shitty workaround for now.
3 jobs for nielsdg/hide-link-button-for-now in 8 minutes and 19 seconds (queued for 11 seconds)