Remove `wsconncache` after migration period

Right now, `wsconncache` is left in for backwards compatibility from https://github.com/coder/coder/pull/8176 deprecating it. Agents used to all listen on the same Wireguard IPV6, but were transitioned to generate unique IPs on startup. Agents and CLIs must be updated before `wsconncache` is removed, otherwise they will become inaccessible.

Migration timeline is TBD.