[llvm-branch-commits] [CI] Cleanup buildkite test report script (PR #143480)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/143480

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.