chore(walredo): avoid duplicate tenant_id and shard_slug fields

[problame]

spotted during reviews of async walredo work in #6628

[github-actions]

3204 tests run: 3065 passed, 0 failed, 139 skipped (full report)

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Flaky tests (4)

Postgres 15

• test_pageserver_restarts_under_worload: release
• test_branched_from_many_empty_parents_size: debug

Postgres 14

• test_subscriber_restart: release
• test_delete_timeline_client_hangup: debug

Code coverage* (full report)

• functions: 31.5% (6598 of 20917 functions)
• lines: 48.5% (51046 of 105214 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
d77484b at 2024-06-06T09:40:24.621Z :recycle:}