Remove legacy Elasticsearch migration workshop from partners

[sishuo-yang]

Summary

• remove the legacy partner_labs/02-elasticsearch-migration-lab workshop from the partners branch
• retire the old Markdown instructions and Part 4 question/answer files after their migration to WorkshopHouse
• avoid duplicate public workshop artifacts now that the maintained learner bundle is submitted under workshops/

Replacement PRs

• hosted playbook and interactive 35-question quiz: https://github.com/ClickHouse/WorkshopHouse/pull/47
• learner artifacts on build-workshop-v1: Publish Elasticsearch migration workshop artifacts #92

Verification

• confirmed all 82 deleted files are confined to partner_labs/02-elasticsearch-migration-lab
• confirmed no references to the removed lab path remain on the branch
• git diff --cached --check

Merge order

Merge the two replacement PRs before this cleanup PR.