ipn/ipnserver: remove IPN protocol server

Unused in this repo as of the earlier #6450 (300aba61a6)
and unused in the Windows GUI as of tailscale/corp#8065.

With this ipn.BackendServer is no longer used and could also be
removed from this repo. The macOS and iOS clients still temporarily
depend on it, but I can move it to that repo instead while and let its
migration proceed on its own schedule while we clean this repo up.

Updates #6417
Updates tailscale/corp#8051

Change-Id: Ie13f82af3eb9f96b3a21c56cdda51be31ddebdcf
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/up.go

@@ -571,14 +571,6 @@ func runUp(ctx context.Context, cmd string, args []string, upArgs upArgsT) (retE
 			}
 			if n.ErrMessage != nil {
 				msg := *n.ErrMessage
-				if msg == ipn.ErrMsgPermissionDenied {
-					switch effectiveGOOS() {
-					case "windows":
-						msg += " (Tailscale service in use by other user?)"
-					default:
-						msg += " (try 'sudo tailscale up [...]')"
-					}
-				}
 				fatalf("backend error: %v\n", msg)
 			}
 			if s := n.State; s != nil {

cmd/tailscale/cli/web.go

@@ -23,7 +23,6 @@ import (
 	"net/url"
 	"os"
 	"os/exec"
-	"runtime"
 	"strings"
 
 	"github.com/peterbourgon/ff/v3/ffcli"
@@ -470,14 +469,6 @@ func tailscaleUp(ctx context.Context, st *ipnstate.Status, forceReauth bool) (au
 		}
 		if n.ErrMessage != nil {
 			msg := *n.ErrMessage
-			if msg == ipn.ErrMsgPermissionDenied {
-				switch runtime.GOOS {
-				case "windows":
-					msg += " (Tailscale service in use by other user?)"
-				default:
-					msg += " (try 'sudo tailscale up [...]')"
-				}
-			}
 			return "", fmt.Errorf("backend error: %v", msg)
 		}
 		if url := n.BrowseToURL; url != nil && printAuthURL(*url) {