QFileSystemModel: remove incorrect ### Qt 7 comment

QTimeZone is not trivially-copyable, so we shouldn't default arguments
of that type (cf. QTBUG-98117), but continue overloading.

Found in API-review.

Task-number: QTBUG-98117
Pick-to: 6.6
Change-Id: I24e9d6c10512a50c172b5722c7d75bf74adf60a6
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/gui/itemmodels/qfilesystemmodel.h

@@ -114,7 +114,6 @@ public:
     qint64 size(const QModelIndex &index) const;
     QString type(const QModelIndex &index) const;
 
-    // ### Qt7 merge the two overloads, with tz QTimeZone::LocalTime
     QDateTime lastModified(const QModelIndex &index) const;
     QDateTime lastModified(const QModelIndex &index, const QTimeZone &tz) const;