Remove deprecated Connect code for old kube sessions and arbitrary SSH hosts

[ravicious]

This PR removes a bunch of deprecated code that I was supposed to remove a long time ago. 🙃

• doc.terminal_tsh_kube provided the old way of connecting to kube clusters. It executed tsh kube login first and then left the session open. This was replaced in v14 by transparently creating a local proxy for a kube cluster before opening a shell session, then just setting KUBECONFIG to a config that points to the local proxy.
• DocumentTshNodeWithLoginHost provided a way to connect to an SSH node in the cluster by doing tsh kube user@host in the command bar. This was removed in v13 and now there's no way to connect to an arbitrary host through Connect's UI anymore.
  • GetServers RPC was used to resolve the host to a specific node in the cluster, as well as to list servers in the cluster. Resource-specific screens were replaced with unified resources. Connections to arbitrary hosts were removed, so the RPC is no longer needed.

The deprecation comments mention migrations, but I don't think they're necessary anymore. doc.terminal_tsh_kube won't blow up, it'll just show a message about the document kind not being supported. DocumentTshNodeWithLoginHost would blow up only if someone executed tsh ssh user@host from the nonexistent command bar in Connect and then closed Connect before the app was able to resolve user@host to a specific SSH node.

[rosstimothy]

Thanks for the clean up! 🧹 🗑️ 🧽

[gzdunek]

Thanks for cleaning this up!

[zmb3]

Amazing. Thank you.