[libcxx] [llvm] [LIT] remove `to_unicode`, `to_string`, and `to_bytes` helpers (PR #165950)

[Tomohiro Kashiwada via llvm-commits]

kikairoya wrote:

All pre-merge CI passed. It seems there was something temporarily flaky.

I've verified that `llvm-test-suite` doesn't use these helpers anymore, so I believe this won't break buildbots.

https://github.com/llvm/llvm-project/pull/165950