Revert my CMake patches concerning building with /MT (r194589, r194596)

Reid pointed out we already have LLVM_USE_CRT_{buildtype} to set it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194604 91177308-0d34-0410-b5e6-96231b3b80d8