release 24.3: backup: remove TestBackupSharedProcessTenantNodeDown

[kev-cao]

Backport 1/1 commits from #159006.

/cc @cockroachdb/release

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This test is flaky and superfluous as our mixed version roachtests already test backup with offline nodes.

Fixes: #158654

Release note: None

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Release justification: Test-only change.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[cockroach-teamcity]

This change is 

[blathers-crl]

✅ PR #159421 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The pull request is compliant as it strictly pertains to test-only changes. The modified file backup_tenant_test.go falls under the category of test files (*_test.go), which are exempt from the main backport policy requirements according to the non-production assessment criteria. Since the changes involve the removal of a test case described as 'flaky and superfluous', this qualifies the PR as adhering to the provided exceptions for non-production file changes in backporting.

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