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/genericprojectmanager/genericbuildconfiguration.h

@@ -96,8 +96,6 @@ class GenericBuildSettingsWidget : public ProjectExplorer::BuildConfigWidget
 public:
     GenericBuildSettingsWidget(GenericBuildConfiguration *bc);
 
-    QString displayName() const;
-
 private slots:
     void buildDirectoryChanged();