effects/private: Drop opacity binding inside WindowHeapDelegate

This targetOpacity is nowhere to be found. This is a relic of past
development. Currently code that needs to alter opacity based on drag
state does so externally by setting opacity on a delegate as a whole.

src/effects/private/qml/WindowHeapDelegate.qml

@@ -122,7 +122,6 @@ Item {
                             thumb.activeDragHandler.centroid.position.y
                     width: cell.width * thumb.targetScale
                     height: cell.height * thumb.targetScale
-                    opacity: thumb.activeDragHandler.targetOpacity
                 }
             }
         ]