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/qmakeprojectmanager/qmakestep.cpp

@@ -616,11 +616,6 @@ QMakeStepConfigWidget::~QMakeStepConfigWidget()
     delete m_ui;
 }
 
-QString QMakeStepConfigWidget::summaryText() const
-{
-    return m_summaryText;
-}
-
 void QMakeStepConfigWidget::qtVersionChanged()
 {
     updateSummaryLabel();
@@ -833,14 +828,6 @@ void QMakeStepConfigWidget::recompileMessageBoxFinished(int button)
     }
 }
 
-void QMakeStepConfigWidget::setSummaryText(const QString &text)
-{
-    if (text == m_summaryText)
-        return;
-    m_summaryText = text;
-    emit updateSummary();
-}
-
 ////
 // QMakeStepFactory
 ////