[ci] Remove unused generate-buildkite-pipeline-scheduled script (#79320)

The "scheduled build" pipeline on BuildKite had been disabled for months
and doesn't exist anymore, so this script is effectively dead code. When
we set up a cron-activated build again, we should do it using Github
actions (which could trigger a BK pipeline if needed).

Keeping this script around just creates additional confusion about
what's used and what's not used for doing CI.