Remove deprecated `Dataset` aliases from `airflow.__init__`

[SHIVANSH-ux-ys]

In airflow-core/src/airflow/__init__.py, there is a TODO comment indicating that the deprecated Dataset exports should be removed in Airflow 3.2. Since the project is now beyond 3.2, we should remove these deprecated aliases to clean up the top-level namespace.

[boring-cyborg]

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