release-25.2: backup: delete TestBackupRestoreWithConcurrentWrites

[msbutler]

Backport 1/1 commits from #170607 on behalf of @msbutler.

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This test has been flaking a ton, and we already have coverage for this in the roachtest.

Fixes #170025

Release note: none

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Release justification:

[trunk-io]

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[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 OR fixes for serious issues. Non-production includes test-only changes, build system changes, etc. Serious issues are 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. Reference the approved ENGREQ ticket in the PR body (e.g., "Fixes ENGREQ-123").

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.

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[cockroach-teamcity]

This change is