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

1. Remove the format-checking job from the BuildKite pipeline. We now
have a monorepo-wide format checker implemented with Github Actions, so
that should not be necessary anymore.

2. Stop building and testing Clang on Windows via the clang-ci pipeline.
We already do that in the github-pull-requests pipeline, so that's just
duplicate work.

3. Stop testing Clang on Linux in the clang-ci pipeline. We already do
that in the github-pull-requests pipeline too, so that's also duplicate
work. For now we still build Clang because the other jobs in the
clang-ci pipeline require its artifacts, but that could be improved.

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