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

[via llvm-commits]

h-vetinari wrote:

> The CMake configuration as a whole is failing, the logs show:

The interesting thing is that everything _past_ that check is failing, which somehow looks like it's corrupting some (CMake's?) state somehow:
```
-- Performing Test CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG
-- Performing Test CXX_SUPPORTS_UNWINDLIB_EQ_NONE_FLAG - Success
-- Performing Test CXX_SUPPORTS_NOSTDLIBXX_FLAG
-- Performing Test CXX_SUPPORTS_NOSTDLIBXX_FLAG - Failed
-- Performing Test CXX_SUPPORTS_NOSTDINCXX_FLAG
-- Performing Test CXX_SUPPORTS_NOSTDINCXX_FLAG - Failed
-- [everything after that fails]
```

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