Remove Designer's "." normaloff file data from icon properties in .ui files

Qt Designer (accidentally) injected "." file names as fallback when
setting theme icon names, which then results in generated code like:
    QIcon icon;
    QString iconThemeName = QString::fromUtf8("edit-paste");
    if (QIcon::hasThemeIcon(iconThemeName)) {
        icon = QIcon::fromTheme(iconThemeName);
    } else {
        icon.addFile(QString::fromUtf8("."), QSize(), QIcon::Normal, QIcon::Off);
    }
Such a "." file name is not documented to serve any purpose, just results
in failing icon lookup.

See QTBUG-115333.

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