release-25.4: 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 the change does not break backwards compatibility and otherwise complies with the backport policy. Include a brief release justification in the PR description explaining why the backport is appropriate. All backports must be reviewed by the TL for the owning area. While the stricter LTS policy does not yet apply, please exercise judgment and consider gating non-critical changes behind a disabled-by-default feature flag when appropriate.

[cockroach-teamcity]

This change is