Remove unused/dead code in migration utils

[Dev-iL]

While working on improving the migration tests, the LLM noted that most functions in the affected file are unused in the codebase - so they're being removed.

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[potiuk]

Pending CI agrees.

[Dev-iL]

Pending CI agrees.

Do we need to run this for "all versions" or add any other tags so we're more confident about the CI's outcome?

[potiuk]

Pending CI agrees.

Do we need to run this for "all versions" or add any other tags so we're more confident about the CI's outcome?

Good idea.

[potiuk]

Added label and rebased to re-trigger the CI.