llvm: remove obsolete workaround

[mistydemeo]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

---

This workaround is no longer needed, and in fact the HEAD build now fails unless this is removed.

cc @Bo98

[Bo98]

Looks like it's because of llvm/llvm-project@61e8973.

In which case, I think we might want to add libcxxabi to the runtimes array (on HEAD only currently)?

(Tagging "do not merge" so automerge doesn't pull this before that question is answered)

[mistydemeo]

Confirmed, adding that to the runtimes array is necessary for the build to complete. Amended, with a comment.