Revert "cmd/tailscaled, ipn/{ipnlocal,ipnserver}: let netstack get access to LocalBackend"

This reverts commit 2bc518dcb2.

@namansood didn't end up needing it in his 770aa71ffb.

ipn/ipnserver/server.go

@@ -92,10 +92,6 @@ type Options struct {
 	// DebugMux, if non-nil, specifies an HTTP ServeMux in which
 	// to register a debug handler.
 	DebugMux *http.ServeMux
-
-	// OnBackendCreated, if non-nil, is called once when the LocalBackend
-	// is created.
-	OnBackendCreated func(*ipnlocal.LocalBackend)
 }
 
 // server is an IPN backend and its set of 0 or more active connections
@@ -743,10 +739,6 @@ func Run(ctx context.Context, logf logger.Logf, logid string, getEngine func() (
 		return smallzstd.NewDecoder(nil)
 	})
 
-	if opts.OnBackendCreated != nil {
-		opts.OnBackendCreated(b)
-	}
-
 	if opts.DebugMux != nil {
 		opts.DebugMux.HandleFunc("/debug/ipn", func(w http.ResponseWriter, r *http.Request) {
 			serveHTMLStatus(w, b)