Clean up non-C++14 code (#28443)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28443
We're now on C++14, so we don't need the else branch of these ifdef's anymore
ghstack-source-id: 94904074
Test Plan: waitforsandcastle
Differential Revision: D18069136
fbshipit-source-id: f1613cab9a99ee30f99775e4a60a1b06fd0a03ff
