Remove PHP 5.6 workaround/comments

[jamieburchell]

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@judgej

Looking at this diff there are comments about a PHP 5.6 specific workaround:

        // The Response in the current namespace conflicts with
        // the Response in the namespace one level down, but only
        // for PHP 5.6. This alias works around it.

Since the library now only supports PHP 7.3+, this should be reviewed for relevance. It's not clear if the comment is still relevant to the surrounding code and the changes that have been made to it.