Fix remaining orphaned data

[jonasraoni]

Describe the bug
The 3.4 release is doing a lot of cleanup operations on the database, but some entities were not fully handled intentionally. And due to the absence of a couple of foreign keys, new orphan data might reappear.
The remaining foreign keys will be covered by #8333.

As part of this issue, it's needed to:

• Clear disk related assets: we cleared issue_galleys, submission_files, etc., but their related files remained alive both in the database and the disk.
• Investigate whether we have other similar cases: independent entities that are considered orphaned when not linked to anything (e.g. the files table).

What application are you using?
OJS 3.4rc3