release-25.4: workload/schemachange: don't predict deps on DROP COLUMN

[spilchen]

Backport 1/1 commits from #169328.

/cc @cockroachdb/release

---

The schemachange workload tried to predict precisely when DROP COLUMN would fail with DependentObjectsStillExist. Keeping that prediction in sync with the schema changer has been a steady source of flakes.

Move the error to potentialExecErrors so it is permitted but not required, and delete the dependency-detection helpers that fed the old prediction.

Resolves #168658

Release note: None
Epic: none

Release justification: test only fix

[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 

[blathers-crl]

Detected infrastructure failure (matched: self-hosted runner lost communication with the server). Automatically rerunning failed jobs. (run link)