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

[Louis Dionne via cfe-commits]

ldionne wrote:

I will merge once the CI has passed. If someone else sees the CI green, feel free to merge too.

@Endilll We could indeed "flatten" the `clang-ci` pipeline into the `github-pull-requests` pipeline. Basically, instead of triggering the `clang-ci` pipeline like this: https://github.com/llvm/llvm-project/blob/main/.ci/generate-buildkite-pipeline-premerge#L213

We would instead expand to the pipeline itself and it would run inside the `github-pull-requests` pipeline itself. It would then be easier to share artifacts between jobs within that pipeline, and the `clang-ci` pipeline could go away entirely.

The same treatment could potentially be applied to libc++ although there will be other challenges (namely generating a valid pipeline YAML inside `.ci/generate-buildkite-pipeline-premerge` may be challenging).

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