Duplicate PSYoung/OldGen max size functions

After obsoleting -XX:UseAdaptiveGCBoundary there is a lingering cleanup to be done around the functions gen_size_limit for both PSYoungGen and PSOldGen. Each of those classes has another function (PSYoungGen::max_size() and PSOldGen::max_gen_size()) that seems to be duplicative of the respective gen_size_limit. One of each pair should be removed, and naming should be regularized.