BuildConfiugration::subConfigWidge() Provide a default implementation

Which simply returns a BuildEnvironmentWidget. A long time ago
BuildConfigurations had no environment, nowdays they do. So it makes
sense for all BuildConfigurations to have the BuildEnvironmentWidget.

Change-Id: I824c45df79a0dcd2b624bf67a4730fb5dab098bc
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/cmakeprojectmanager/cmakebuildconfiguration.h

@@ -52,7 +52,6 @@ public:
     ~CMakeBuildConfiguration();
 
     ProjectExplorer::BuildConfigWidget *createConfigWidget();
-    QList<ProjectExplorer::BuildConfigWidget *> subConfigWidgets();
     QString buildDirectory() const;
 
     void setBuildDirectory(const QString &buildDirectory);