[libunwind] [CMake] Remove leftover no-op cmake variable setting. NFC.

The setting and restoring of this variable became unused in
3ee0cec88effc88285732c8bec2a8f0e4e37c0b1 / D112155.

Differential Revision: https://reviews.llvm.org/D124372

NOKEYCHECK=True
GitOrigin-RevId: 7164c5f051a980442f2376af4bae3740710bf347
1 file changed
tree: 8b072ee27a31f54b837285931cd8ebdea0f50f57
  1. cmake/
  2. docs/
  3. include/
  4. src/
  5. test/
  6. .clang-format
  7. CMakeLists.txt
  8. LICENSE.TXT