Remove ActionLevels remnant uses
ActionLevels were removed in commit 4a9fae4e but were still references in few cases.
This commit removes all remaining uses.
First was a prefetch when generating CSV and YAML for a given
TestJob results. Apparently Django allows you to try to prefetch
non-existing fields. Replace prefetch with select_related as that
gives 10% performance boost on top.
Second it was referenced in fields to remove when exporting query.
However, actionlevels is no longer part of any model.