Cleanup outdated comment about gcc 5.3
We have some occurrences of this sentence for some introspection helpers that have been copied around: for g++ > 5.3, this can be replaced by a lambda
These comments are outdated. We don't support 5.3 anymore but also I think we checked and using lambda is not that easy in this context. If there are more comments of this sort (in other places), we can also make a cleanup commit afterward.