[libcxx-commits] [libcxx] [libc++][NFC] Remove redundant define (PR #215883)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 requested changes to this pull request.

The point of this is that it removes parsing errors later in the file when it's compiled standalone (e.g. when opened in an IDE). The `#error` doesn't actually affect how the file is parsed except for a diagnostic on that one line.

https://github.com/llvm/llvm-project/pull/215883