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

[Louis Dionne via llvm-commits]

https://github.com/ldionne approved this pull request.

BTW this LGTM if we can make it work -- this is a great cleanup, thanks for noticing.

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