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

[via llvm-commits]

h-vetinari wrote:

Hm, this seems to affect some feature detection (and constraints) later on, though I don't see how:
```
 CMake Error at /home/runner/_work/llvm-project/llvm-project/libunwind/src/CMakeLists.txt:102 (message):
  Compiler doesn't support generation of unwind tables if exception support
  is disabled.  Building libunwind DSO with runtime dependency on C++ ABI
  library is not supported.
```

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