Project scoped issue types become orphaned on project deletion

Issue Summary

The creation of NextGen projects results in certain types of issue types being created and scoped to the project. However, the deletion of issue types does not result in the issue types scoped to the project from being deleted.

Steps to Reproduce

1. Query the issue types: https://example.atlassian.net/rest/api/3/issuetype
2. Create a NextGen project.
3. Query the issue types: https://example.atlassian.net/rest/api/3/issuetype
4. Move the project to trash.
5. Query the issue types: https://example.atlassian.net/rest/api/3/issuetype

Expected Results

At step 5, the returned issue types are the same as those from step 1.

Actual Results

At step 5, the returned issue types are the same as those from step 3, except without the scoping to the deleted project thus causing them to be indistinguishable from certain other non project scoped issue types.

Workaround

None