If you visit the list of screens, each one will be shown with a list of places that it is used. Two columns, one for "screen schemes" and the other for "workflow". If both columns are empty, it is safe to delete the screen.
Ok, I think you've run into a problem where JIRA's workflows lose their screen associations.
Have a look at https://confluence.atlassian.com/jirakb/no-delete-option-on-screens-432801027.html (obviously, you're a bit limited on Cloud and could quite easily punt this over to Atlassian support)
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