release-25.2: workload: remove FK dependency check in schemachanger drop column

[spilchen]

Backport 1/1 commits from #159188.

/cc @cockroachdb/release

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The schemachanger previously attempted to predict whether a DROP COLUMN operation would fail due to a foreign key (FK) dependency. This logic has proven to be unreliable and difficult to maintain correctly.

Rather than investing more effort into fixing the latest edge cases, we’ve decided to remove the FK dependency detection logic altogether. This simplifies the codebase.

Resolves: #159034 Release note: none

Release justification: test only fix that will improve stability of schemachanger workload

[blathers-crl[bot]]

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.

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[cockroach-teamcity]

This change is 

[blathers-crl[bot]]

✅ PR #159254 is compliant with backport policy

Confidence: high Backward compatible: true Explanation: This PR is compliant with the backport policy as it falls under the exceptional categories outlined in the policy. The PR is specifically described in its body as a 'test only fix' and the affected files are within the 'pkg/workload/schemachange/', which is designated as part of the development tools and testing infrastructure. According to the CockroachDB Backport Policy, changes that only affect non-production code, tests, or development tools are exempt from the standard backport policy requirements, which means they do not require a feature flag or a critical bug fix justification beyond being a test fix or improvement to stability.

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