Delete image attachments linked to deleted ``ir.ui.view`` records only when
no other view's HTML references them via ``/web/image/<id>`` or
``/web/content/<id>``. This avoids the aggressive approach of deleting all
``res_id = 0`` attachments, which can break website images that survived
their parent view's deletion but are still embedded in live pages.
The ``NOT EXISTS`` subquery scans all ``arch_db`` columns and is expensive
but runs once at migration time.
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