As a possible punt… I believe deleting a topic now automatically removes the solution and deletes the record in discourse_solved_solved_topics. I’m not 100% that this could work without the solved plugin installed, but it could be worth a check?
If you run a query in data explorer to grab them all, delete one, and run the query again to see if it’s disappeared, that could be an easy way to see if it could work?
Restoring the topic doesn’t restore the record in discourse_solved_solved_topics, so (potentially 
) a quick ‘delete and restore’ could clear them out. 