Remove deployments bigint conversion artifacts

Summary

Complete the release N+2 cleanup for the deployments table only.

The four canonical columns are bigint:

• id
• environment_id
• project_id
• user_id

The cleanup is implemented by !252750 (merged) in milestone 19.4. Production and staging evidence is recorded in #611715.

Acceptance criteria

• Remove id_convert_to_bigint, environment_id_convert_to_bigint, project_id_convert_to_bigint, and user_id_convert_to_bigint.
• Remove bigint synchronization trigger trigger_a2cc844b73ed.
• Remove only temporary database objects that belong to the bigint conversion.
• Update db/integer_ids_not_yet_initialized_to_bigint.yml through the migration helper.
• Keep db/structure.sql unchanged because new installations already use the final schema.
• Verify the cleanup after deployment.
• Leave Deployment ignore-rule removal to #551606 (closed).

Cleanup for deployment_clusters is tracked in #627120 (closed). The deployment_merge_requests conversion has not reached the cleanup stage and is tracked in #581419.