window: Remove default drop target style

The default does not fit well with our new custom one.

This also removes a bunch of code which was not used.

Part-of: <https://gitlab.gnome.org/GNOME/loupe/-/merge_requests/258>

src/window.rs

@@ -419,7 +419,6 @@ mod imp {
                     true
                 }),
             );
-            self.drag_overlay.set_drop_target(&*self.drop_target);
         }
     }