Remove conditional from `onRejected`

[Casey McLaughlin]

Refactor this block to assume that the Create class exists:

            // ...

            // If made it here, then we have decided not to retry the request
            // Future-proofing this; remove when bumping minimum Guzzle version to 7.0
            if (class_exists('\GuzzleHttp\Promise\Create')) {
                return \GuzzleHttp\Promise\Create::rejectionFor($reason);
            } else {
                return rejection_for($reason);
            }

This will require updating the Guzzle dependency to v7, so this can't happen for a while.