Hi Maria,
I can see only one option, as deleting attachments is not possible directly in workflow global scope action.
1. Create a technical path on each step, which should be taken into consideration for deleting attachments.
2. To that path add an action (from template) - you could delete attachment and historical binary data or only historical binary data (this would leave latest version in the system).
3. Create a global action, which would use 'Move workflow (SQL)' action. Within select specify instances which should have attachments deleted. I'd recommend preparing it in a way, that it will run on X instances only, but few times, as processing more than 50 documents in one batch might end up with timeouts.
Instead of global action, you could also write some script which would get data from SQL, and move instances using webcon REST API - this way you can avoid timeouts, but it requires more work.
In case of already deleted attachments I'm not sure, but sounds like it could require direct update on database, and before doing it globally, i'd try to check how it works on one element on dev env if possible. Manual updates are against best practices, but sometimes it's the only solution and even webcon does it if necessary.
You could also think about using archive option, which would delete document from WEBCON, but it's more than just removing attachments.
A workaround could be an action that will clone existing document as new one, and delete the old one, but it's not the best solution.