Remove GCC4 from CI (#26522)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26522
Our binaries are already built using GCC5, so there is no reason to test for GCC4 anymore.
This is an important prerequisite for switching to C++14, but even without the C++14 switch, this enables a gcc feature that I need for constexpr type ids.
ghstack-source-id: 91851144
Test Plan: unit tests
Differential Revision: D17494507
fbshipit-source-id: 7c0beb5e532ad9caa5cb02c1af26341c1017ff57
