ProjectExplorer: Remove pure ProjectConfiguration::isActive function

It's never used polymorphically, and only caused an unused
implementation in BuildStep.

Change-Id: I5fbd34925788ca22960a2f522eef75be3137c67b
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/projectexplorer/buildconfiguration.h

@@ -103,7 +103,7 @@ public:
 
     static QString buildTypeName(BuildType type);
 
-    bool isActive() const override;
+    bool isActive() const;
 
     static void prependCompilerPathToEnvironment(Kit *k, Utils::Environment &env);
     void updateCacheAndEmitEnvironmentChanged();