[CI] Cleanup buildkite test report script
This patch removes the generate_test_report_buildkite script entrypoint
as we no longer need it now that buildkite has been sunsetted. Also
remove the calls in the monolithic-* scripts given they are adding
complexity for no value.
Also remove the generate-buildkite-pipeline-premerge script as it is
no longer needed.
Reviewers: tstellar, DavidSpickett, lnihlen, cmtice
Reviewed By: DavidSpickett
Pull Request: https://github.com/llvm/llvm-project/pull/143480
