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

[via llvm-commits]

h-vetinari wrote:

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

Yeah, it looks like the picolib-noexceptions build is _still_ trying to actually build the exceptions, and then fails to find the symbols when linking. I've tried looking for anything that would switch arm/pico or the `"-mthumb"` that's  in [`Armv7Thumb-no-exceptions.cmake`](https://github.com/llvm/llvm-project/blob/main/libcxx/cmake/caches/Armv7Thumb-no-exceptions.cmake) which sets the baseline config for that job.

I do not think I'll be able to fix this without some help, or at least pointers.

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