QPrintSupport: do not bypass a base class' virtual

QAbstractPrintDialog (a QDialog subclass) has an interesting
override of exec(): a pure virtual, without a body.
The UNIX subclass was therefore forced to override it, but
since it did not need to do anything with it, it had to call
QDialog::exec (bypassing the direct base, otherwise it
would cause a pure virtual call).

Eliminate the pure virtual override. This should be BC;
the layout of the vtable does not change, merely its contents.

Change-Id: I84ac23c938f1934f699df032ef1bde0d6df77784
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/printsupport/dialogs/qabstractprintdialog.h

@@ -84,8 +84,6 @@ public:
     explicit QAbstractPrintDialog(QPrinter *printer, QWidget *parent = nullptr);
     ~QAbstractPrintDialog();
 
-    int exec() override = 0;
-
     // obsolete
     void addEnabledOption(PrintDialogOption option);
     void setEnabledOptions(PrintDialogOptions options);

src/printsupport/dialogs/qprintdialog_unix.cpp

@@ -1082,7 +1082,7 @@ void QPrintDialog::setVisible(bool visible)
 
 int QPrintDialog::exec()
 {
-    return QDialog::exec();
+    return QAbstractPrintDialog::exec();
 }
 
 void QPrintDialog::accept()