Drop deprecated alternative Browser constructor argument order

[clue]

This simple changeset drops the deprecated alternative Browser constructor argument order. As of v1.5.0, we recommend using the new Browser signature with the optional $connector as first argument and optional $loop as second argument, so upgrading should be straightforward for most use cases:

// unchanged$browser = newReact\Http\Browser();
// old (no longer supported)$browser = newReact\Http\Browser(null, $connector);
$browser = newReact\Http\Browser($loop, $connector);
// new (already supported since v1.5.0)$browser = newReact\Http\Browser($connector);
$browser = newReact\Http\Browser($connector, $loop);

The original deprecation message has been in place for a couple of years already, so this should be relatively safe to apply.

Note that this temporarily introduces an implicitly nullable type that should be removed in a follow-up as discussed in reactphp/promise#260. Additionally, a follow-up PR should drop the entire $loop argument as discussed in #517.

Builds on top of #418, #528 and reactphp/socket#315.

[SimonFrings]

Nice, looks good to me 👍