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

[Louis Dionne via llvm-commits]

ldionne wrote:

I think something else is going on. The CMake configuration as a whole is failing, the logs show:

```
-- Performing Test CXX_SUPPORTS_NOSTDLIBXX_FLAG - Failed
-- Performing Test CXX_SUPPORTS_NOSTDINCXX_FLAG
-- Performing Test CXX_SUPPORTS_NOSTDINCXX_FLAG - Failed
```

This shouldn't be affected by this patch, but somehow it is.

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