Remove obsolete policy settings

Summary:
The line

 cmake_minimum_required(VERSION 3.4.3)

already has the effect of setting to NEW all policies present in that
release:

 https://cmake.org/cmake/help/v3.4/manual/cmake-policies.7.html

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D50407

llvm-svn: 339376