Revert "[libc++] [P0879] constexpr std::nth_element, and rewrite its tests."

This reverts commit 207d4be4d9d39fbb9aca30e5d5d11245db9bccc1 due to returning incorrect results. Regression test case posted in D96074.

GitOrigin-RevId: b6ffece32035a90d181101f356bd9c04ea1d3122
3 files changed