Remove unused FAB app-init helpers that could never run

[FrankYang0529]

Remove three leftovers of the Airflow 2 Flask webserver from the FAB provider: init_appbuilder_views(), cached_app() / purge_cached_app(), and the two conftest entries that referenced init_appbuilder_views().

Why

• init_appbuilder_views() has had no caller since Remove references of "airflow.www" in FAB provider #46914 copied it into the provider. The only view it registers, FabIndexView, is already added by AirflowAppBuilder._add_admin_views() (since Make FAB auth manager login process compatible with Airflow 3 UI #45765).
• cached_app() calls create_app() without the required enable_plugins argument, so it raises TypeError on first use, and purge_cached_app() only ever cleared that always-empty cache. Airflow 3 mounts the Flask app object into FastAPI directly and has no use for a gunicorn-style cached factory.

Verification

• uv run --project providers/fab pytest providers/fab/tests/unit/fab/www --with-db-init
• uv run --project providers/fab pytest providers/fab/tests/unit/fab/auth_manager --with-db-init

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

• Yes - Claude Code

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[vincbeck]

Love that