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/psiphon_test.go

@@ -67,14 +67,3 @@ func TestPsiphonStartFailure(t *testing.T) {
 		t.Fatal("expected nil tunnel here")
 	}
 }
-
-func TestPsiphonNilTunnel(t *testing.T) {
-	var tunnel *psiphonTunnel
-	if tunnel.BootstrapTime() != 0 {
-		t.Fatal("expected zero bootstrap time")
-	}
-	if tunnel.SOCKS5ProxyURL() != nil {
-		t.Fatal("expected nil SOCKS Proxy URL")
-	}
-	tunnel.Stop() // must not crash
-}