Remove TenantState::Loading

[hlinnaka]

The last real use was removed in commit de90bf4. It was still used in a few unit tests, but they can use Attaching too.

[github-actions]

5012 tests run: 4848 passed, 0 failed, 164 skipped (full report)

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Flaky tests (7)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_multi_attach: release-x86-64

Postgres 16

• test_neon_cli_basics: release-arm64
• test_pg_regress[4]: release-arm64
• test_subscriber_restart: release-x86-64

Postgres 15

• test_subscriber_restart: release-x86-64

Postgres 14

• test_physical_replication: release-x86-64

Code coverage* (full report)

• functions: 32.1% (7457 of 23231 functions)
• lines: 49.9% (60090 of 120371 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
b6f186f at 2024-09-24T21:09:08.047Z :recycle:}