Revert backfill on ci_build_trace_sections

Andreas Brandl requested to merge ab/revert-ci-build-trace-sections into master

What does this MR do?

We're seeing problems for (small) self-managed instances with the backfill migration for ci_build_trace_sections. Since we also intend to drop this table anyways (#32565 (closed)), we don't really need to run this migration anymore.

This change disables the migration and actively removes the backfill (batched background) migration if the record is present.

Edited by Andreas Brandl

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