ipn,wgengine: remove vestigial Prefs.AllowSingleHosts

It was requested by the first customer 4-5 years ago and only used
for a brief moment of time. We later added netmap visibility trimming
which removes the need for this.

It's been hidden by the CLI for quite some time and never documented
anywhere else.

This keeps the CLI flag, though, out of caution. It just returns an
error if it's set to anything but true (its default).

Fixes #12058

Change-Id: I7514ba572e7b82519b04ed603ff9f3bdbaecfda7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tsconnect/wasm/wasm_js.go

@@ -298,11 +298,10 @@ func (i *jsIPN) run(jsCallbacks js.Value) {
 	go func() {
 		err := i.lb.Start(ipn.Options{
 			UpdatePrefs: &ipn.Prefs{
-				ControlURL:       i.controlURL,
-				RouteAll:         false,
-				AllowSingleHosts: true,
-				WantRunning:      true,
-				Hostname:         i.hostname,
+				ControlURL:  i.controlURL,
+				RouteAll:    false,
+				WantRunning: true,
+				Hostname:    i.hostname,
 			},
 			AuthKey: i.authKey,
 		})