[libcxx] [llvm] [libc++] Remove the explicit Clang/LSV CI job (PR #131667)

[Louis Dionne via llvm-commits]

ldionne wrote:

No, you're right, I think we should instead switch the LSV job to use C++17 instead of C++23 to get back our coverage.

https://github.com/llvm/llvm-project/pull/131667