Drop deprecated alternative Connector constructor argument order

[clue]

This simple changeset drops the deprecated alternative Connector constructor argument order. As of v1.9.0, we recommend using the new Connector signature with the optional $context as first argument and optional $loop as second argument, so upgrading should be straightforward for most use cases:

// unchanged$connector = newReact\Socket\Connector();
// old (no longer supported)$connector = newReact\Socket\Connector(null, $context);
$connector = newReact\Socket\Connector($loop, $context);
// new (already supported since v1.9.0)$connector = newReact\Socket\Connector($context);
$connector = newReact\Socket\Connector($context, $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 #312.

Builds on top of #264 and #314.

[SimonFrings]

Nice, let's get this shipped 👍