BuildConfigWidget: Make displayName property instead of a virtual func

To make the interface the same as NamedWidget.

Change-Id: I5961b4e5aa7f5acf3a7f83e82b6fa4266b9ebf97
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/cmakeprojectmanager/cmakeproject.h

@@ -229,7 +229,6 @@ class CMakeBuildSettingsWidget : public ProjectExplorer::BuildConfigWidget
     Q_OBJECT
 public:
     CMakeBuildSettingsWidget(CMakeBuildConfiguration *bc);
-    QString displayName() const;
 
 private slots:
     void openChangeBuildDirectoryDialog();