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/tor_test.go

@@ -43,17 +43,6 @@ func TestTorTunnelNonNil(t *testing.T) {
 	}
 }
 
-func TestTorTunnelNil(t *testing.T) {
-	var tun *torTunnel
-	if tun.BootstrapTime() != 0 {
-		t.Fatal("not the bootstrap time we expected")
-	}
-	if tun.SOCKS5ProxyURL() != nil {
-		t.Fatal("not the url we expected")
-	}
-	tun.Stop() // ensure we don't crash
-}
-
 func TestTorStartWithCancelledContext(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel() // fail immediately