disable the gitlab runner's session server

We're getting 409 responses from the runner right now, which is preventing it from picking up any jobs. There's an upstream issue for this on gitlab's end [here][1]. From reading that, it looks like the problem is that the they recently enabled verification for whether the advertise_address value for the session server is a public IP. Theorically, we could fix this by making the session server publicly routable, or by enabling local requests for web hooks/services. It's easier to just remove the session server entirely though, since it's only used for the interactive web terminal, which none of us use.