Removed CMake cache upgrade code from 2011

Summary:
This code was added in r141266 to make a breaking change to CMake, but
still be compatible with existing cache files.  The cache files from
2011 are irrelevant today in 2019.

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60711

llvm-svn: 358482