sqlbrowser example: use = default on empty dtors

Idiomatic C++11 code.

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

Change-Id: Iba1fb9874bd93b4a560c33e3ecf62ecaa96d8bda
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 19564e033cf65a0a67680285d1cbcc85c9437aaf)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit c806d2bbbcf38fe0041defb671483d280d23644a)

examples/sql/sqlbrowser/connectionwidget.cpp

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