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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339376 91177308-0d34-0410-b5e6-96231b3b80d8