Remove dead airflow-ctl/newsfragments directory (unused by changelog tooling)

[potiuk]

airflow-ctl/newsfragments/ contained a config.toml declaring towncrier categories, but the breeze release-management generate-airflowctl-changelog command does not consume newsfragments — it builds the changelog section directly from git log over airflow-ctl/. Contributors adding *.significant.rst / *.bugfix.rst fragments there never saw them show up in a release, and stale fragments (e.g. the 59850.significant.rst from 0.1.2 that survived until the 0.1.4 cleanup) silently misled anyone reading the folder.

Rather than wire up a second changelog pipeline, this PR removes the dead directory outright so the folder's existence stops implying a contract the tooling does not honour. If we later decide to switch to fragment-based changelogs, we can reintroduce the folder alongside the tooling change.

No runtime or doc code referenced this folder — the only documentation mention is in dev/README_RELEASE_AIRFLOWCTL.md which already notes that the generator does not consume newsfragments.

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

• Yes — Claude Opus 4.7 (1M context)

Generated-by: Claude Opus 4.7 (1M context) following the guidelines