refactor(tunnel): remove nil tunnels hack (#296)

* refactor(tunnel): remove nil tunnels hack

This code was originally introduced because a tunnel could be
nil in session.go. I have verified that every invocation of
tunnel.Start is careful to ensure that we have a tunnel name
and that we don't manipulate a nil tunnel.

For this reason, I'd rather remove this tricky bit of code and
further simplify the tunnel code.

Part of https://github.com/ooni/probe/issues/985

* even better docs

internal/engine/tunnel/tunnel_test.go

@@ -17,8 +17,8 @@ func TestStartNoTunnel(t *testing.T) {
 			MockableLogger: log.Log,
 		},
 	})
-	if err != nil {
-		t.Fatal(err)
+	if !errors.Is(err, ErrUnsupportedTunnelName) {
+		t.Fatal("not the error we expected", err)
 	}
 	if tunnel != nil {
 		t.Fatal("expected nil tunnel here")