Fix flake tests under asan

ShapeGroupContainerModel::remove() will call KoShapeGroup::invalidateSizeCache(),
even when the shape group is going to be destroyed. This method is called after
KoShapeGroup's destructor, so at this time we no longer have access to members of
KoShapeGroup, i.e. its d-pointer. So this call will cause access violation as the
instance QScopedPointer<Private> d in KoShapeGroup has already been destroyed.

This is a result of getting rid of derived d-pointers, because if we use a derived
d-pointer, it only gets destroyed in the ancestor class (KoShape). We temporarily
fix this problem by manually call model()->deleteOwnedShapes() in KoShapeGroup's
destructor.