ProjectExplorer: Change BuildStepConfigWidget::summaryWidget()

From a virtual function to a normal one backed by a real data member.

That's essentially what several re-implementations did, the other
ones used a fixed value instead.

Change-Id: I61e45f1d4f7f0f80fe2eb1f2729785f37e7bb803
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/ios/iosdeploystepwidget.h

@@ -43,8 +43,6 @@ public:
     ~IosDeployStepWidget();
 
 private:
-    QString summaryText() const override;
-
     Ui::IosDeployStepWidget *ui;
     IosDeployStep *m_step;
 };