Drop trigger, FK, and index for bulk_import_exports.organization_id

What does this MR do and why?

Follow-up to #600457 (closed). Rutger pointed out on !243557 (closed) that organization_id isn't actually needed on bulk_import_export_batches, export_id already makes things cell-safe. Same reasoning applies to the parent table, so we're walking back the sharding key work here.

This is step 1 of 3: stop the app from writing the column, drop the trigger, FK, and index. The column itself stays for now, the physical drop happens in a follow-up once this deploys (dropping a column an app still expects breaks rolling deploys).

Also stops and removes the BackfillBulkImportExportsOrganizationId batched background migration. It finished running but was never finalized, so this follows the "stop and remove" pattern from the BBM docs rather than finalizing something we're about to undo.

One thing worth a second look: I kept db/docs/batched_background_migrations/backfill_bulk_import_exports_organization_id.yml around. The stop-and-remove doc pattern doesn't mention deleting the dictionary YAML, only the finalize-then-squash path does that. Flagging in case a reviewer disagrees.

References

• #604997 (closed)
• #600457 (closed)
• !241088 (merged), !241089 (merged) (the original rollout)
• !243557 (closed) (where Rutger's finding came up)

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Related to #604997 (closed)