[AIP-94] airflowctl variables: add delete command

[FrankYang0529]

Add airflowctl variables delete.

• related: [AIP-94] airflowctl command migration #68402

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

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Generated-by: [Claude Code with Opus 4.8] following the guidelines

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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]

Looks good but would love to get @bugraoz93 call :)

[FrankYang0529]

Close with new PR #68932.