Cleaner query for image cleanup

While working on !1967 (merged), I found out that Exists can now be used directly as a filter, and not only as an annotation as it was restricted to a long time ago. This can be a performance optimization as Django no longer includes the EXISTS () in the SELECT clause, and only uses it in the WHERE clause.

I looked for existing uses of Exists and only found the one in manage.py cleanup to find images without elements. We were using a hack where the annotation was overwritten after filtering on it, specifically to avoid selecting the EXISTS and getting a nice performance boost on that query as PostgreSQL can use a rare and fast Nested Loop Anti Join instead of a long Hash Join. So this merge request changes nothing apart from making the query look a little cleaner.