Why don't you click on the down number on top of airwave and click on "Expand Folders to show all APs and devices". That will show all down APs.
Next, you should create a new view by clicking on the Down Symbol:
Name the view and add a column for "Last Contacted":
After you apply that view, sort by the "last contacted" column to see what APs have been down the longest:
Click on the "Pencil" to select APs and change the dropdown to delete devices: