[libunwind] [llvm] [runtimes] remove workaround for old CMake when setting `--unwindlib=none` (PR #93429)

[Louis Dionne via llvm-commits]

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

Actually, I think some of the libunwind failures on BuildKite are legitimate. See https://buildkite.com/llvm-project/libcxx-ci/builds/35920.

https://github.com/llvm/llvm-project/pull/93429