ProjectExplorer: Remove one unused Target::subscribeSignal overload

Change-Id: I01df947a1dbd8b0518053dc1db528725c48de820
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/projectexplorer/target.h

@@ -128,15 +128,6 @@ public:
         }, recv, this);
     }
 
-    template<typename S, typename R, typename T>
-    void subscribeSignal(void (S::*sig)(), R*recv, T sl) {
-        new Internal::TargetSubscription([sig, recv, sl, this](ProjectConfiguration *pc) {
-            if (S* sender = qobject_cast<S*>(pc))
-                return connect(sender, sig, recv, sl);
-            return QMetaObject::Connection();
-        }, recv, this);
-    }
-
 signals:
     void targetEnabled(bool);
     void iconChanged();