QItemSelectionModel: remove some redundant code

QObject::disconnect(connection) already sets connection.d_ptr to
nullptr, and the default constructor only does that:
QMetaObject::Connection::Connection() : d_ptr(nullptr) {}

So calling disconnect() suffices in this case.

Pick-to: 6.6 6.5 6.2
Change-Id: Icd5f9a15c83939eeecd9efb474ac3b38c0f2dfe4
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

src/corelib/itemmodels/qitemselectionmodel.cpp

@@ -595,10 +595,8 @@ void QItemSelectionModelPrivate::initModel(QAbstractItemModel *m)
 
 void QItemSelectionModelPrivate::disconnectModel()
 {
-    for (auto &connection : connections) {
+    for (auto &connection : connections)
         QObject::disconnect(connection);
-        connection = QMetaObject::Connection();
-    }
 }
 
 /*!