Blender Git Loki

USD Exporter: removed exclusion of 'Holdout' collections

This is in response to @brecht's remark in rBec62413f803e, where he
states that the approach was problematically interpreting the holdout
setting in a different way than what it was designed to do.

If we later want to add back a different "never include this in exports"
criterion, it can be easily done in
`AbstractHierarchyIterator::mark_as_weak_export()`. If such a criterion
should be file-format-specific, it can be done by overriding that
function in the file-format-specific subclass.