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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> Thanks a lot for the fix, I've pulled it into the PR (also rebased on main while I'm at it) - and CI didn't fail as previously, so this looks great! Thank you!

Ok, good! Btw, it might be good to add a reference to https://gitlab.kitware.com/cmake/cmake/-/issues/23454 in the new commit message too (or perhaps even in a comment in libunwind/CMakeLists.txt?), to clarify that this is indeed not a bug but expected behaviour.

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