[clang] [clang][ci] Remove unnecessary BuildKite jobs for Clang (PR #93233)

[Louis Dionne via cfe-commits]

ldionne wrote:

> Do we even need to build Clang and run libc++ jobs? I don't even see them linked in libc++ PR, even besides the fact that I think libc++ is fully tested with GitHub Actions.

What we're testing is that the changes to Clang didn't break libc++. We're testing Clang via libc++ -- we're not testing libc++ itself. So yes, this is still relevant.

https://github.com/llvm/llvm-project/pull/93233