Revert "[VFS] Add 'overlay-relative' field to YAML files" and "[VFS] Fix call to getVFSFromYAML in unittests"

This reverts commit r261552 and r261556 because of failing unittests on
windows:

Failing Tests (4):
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.CaseInsensitive
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.DirectoryIteration
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.MappedFiles
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.UseExternalName

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261613 91177308-0d34-0410-b5e6-96231b3b80d8