REMOVE old QT<= 5.14 code

Ubuntu 22.04  use qt 1.15.3.
In the code is still used qt code <5.10.
A cleanup was done by removing qT code version used in ubuntu 18.04.

src/Gui/Dialogs/DlgCheckableMessageBox.cpp

@@ -172,13 +172,7 @@ void DlgCheckableMessageBox::setText(const QString &t)
 
 QPixmap DlgCheckableMessageBox::iconPixmap() const
 {
-#if QT_VERSION >= QT_VERSION_CHECK(5,15,0)
     return m_d->ui.pixmapLabel->pixmap(Qt::ReturnByValue);
-#else
-    if (const QPixmap *p = m_d->ui.pixmapLabel->pixmap())
-        return QPixmap(*p);
-    return QPixmap();
-#endif
 }
 
 void DlgCheckableMessageBox::setIconPixmap(const QPixmap &p)