Remove temp mode-filter index on ch_sync_events table

What does this MR do and why?

Removes the temporary index_ci_finished_build_ch_sync_events_on_mode_filter partitioned index from p_ci_finished_build_ch_sync_events.

This index was added in !226067 (merged) to support the mode-split sync workers introduced alongside the BackfillCiFinishedBuildsToClickHouse BBM. Since the BBM was finalized in 19.0 and the mode-split worker logic is being removed in the sibling MR (issue #592894), this index is no longer needed.

The remaining index_ci_finished_build_ch_sync_events_for_partitioned_query index continues to cover the non-mode-split sync path.

Closes #592895 (closed)

Database

• New post-deployment migration: db/post_migrate/20260619175737_remove_mode_filter_index_from_ci_finished_build_ch_sync_events.rb
• Uses remove_concurrent_partitioned_index_by_name with disable_ddl_transaction!
• down recreates the index with add_concurrent_partitioned_index (identical to the original migration in !226067 (merged))

Migration up

== 20260619175737 RemoveModeFilterIndexFromCiFinishedBuildChSyncEvents: migrating
== 20260619175737 RemoveModeFilterIndexFromCiFinishedBuildChSyncEvents: migrated (0.0284s)

Migration down (verified reversible)

== 20260619175737 RemoveModeFilterIndexFromCiFinishedBuildChSyncEvents: reverting
== 20260619175737 RemoveModeFilterIndexFromCiFinishedBuildChSyncEvents: reverted (0.1746s)

Related MRs

• Adds index: !226067 (merged)
• Mode-split MR: !226068 (merged)
• Sibling cleanup MR (BBM code removal): TBD (follows from #592894)

MR acceptance checklist

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