Simplify the way we retrieve files in diskFS

The standard lib comes with some powerful tooling we can leverage one of
those to easly get a list of files in a sub-directory of the bucket.
This frees up responsiblity to find the right directory to walk and we
can remove the walk logic entirely which always comes with some
cognitive burden.
Small adjustements

* consistent ordering
* consistent access of params
* use more common NoOp name
* remove prefix handling in sorting