Delta Keyer not working with Clean Plate
With one exception (Garbage Matte), the Delta Keyer doesn't "remove" things.  What it does is create and/or apply masks and mattes so that keyed areas can be replaced by other things (either within the given Delta Keyer node or downstream in the Fusion composition).  Therefore, to get an effect you want, you need to provide the Delta Keyer with an image or a solid color that you want to use to replace the area of the key you have defined.