Remove ImportedFile when making a version no active

[stsewd]

What's the problem this feature will solve?

When a version is marked as inactive, we delete their files from storage,
so there is no need to track those in the ImportedFile table.

Describe the solution you'd like

Delete those files after the version has been marked as inactive. We should also delete existing imported files from not built versions.

qs = ImportedFile.objects.filter(version__built=False)
qs.count()
qs.delete()

Additional context

#10681