Revert backfill on ci_build_trace_sections

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.