sqlbrowser example: use = default on empty dtors

Idiomatic C++11 code.

Amends 2690822428deec4f0c08f4d118d69a7c6036369e, which, however,
inherited the issue from older code.

Pick-to: 6.9 6.8
Change-Id: Iba1fb9874bd93b4a560c33e3ecf62ecaa96d8bda
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

examples/sql/sqlbrowser/connectionwidget.cpp

@@ -34,9 +34,7 @@ ConnectionWidget::ConnectionWidget(QWidget *parent)
 }
 
 ConnectionWidget::~ConnectionWidget()
-{
-}
-
+    = default;
 
 void ConnectionWidget::refresh()
 {