Remove ref assignment

[sjadema]

This is more of a feature request. We're migrating our app from PHP 5.5 => PHP 7+ and are checking if we use incompatible libraries. While running a search for changed variable handling we found the following line in this library: https://github.com/ezyang/htmlpurifier/blob/a93250f251350e306494af1ac3678d2cd36861d7/library/HTMLPurifier/HTMLModule/Tidy.php#L150

This is a false positive since it's commented but the weird reference assignment could be fixed if you read http://php.net/manual/en/migration70.incompatible.php#migration70.incompatible.variable-handling.